Transcriber's Note
Emphasized text is denoted by =Bold= and _Italics_.
MIGRATION OF BIRDS
[Illustration]
Circular 16. Revised Edition - 1979
FISH & WILDLIFE SERVICE / UNITED STATES DEPARTMENT OF THE INTERIOR
MIGRATION OF BIRDS
By Frederick C. Lincoln
Revised By Steven R. Peterson
Associate Editor Peter A. Anastasi
Illustrated By Bob Hines
[Illustration]
Circular 16. Revised Edition - 1979
FISH & WILDLIFE SERVICE / UNITED STATES DEPARTMENT OF THE INTERIOR
TABLE OF CONTENTS
Page
PREFACE 1
INTRODUCTION 2
THE HISTORY AND SCOPE OF MIGRATION 4
TECHNIQUES FOR STUDYING MIGRATION 7
Direct Observation 7
Aural 8
Preserved Specimens 8
Marking 8
Bands, Collars, Streamers 8
Radio Tracking 10
Radar Observation 10
Laboratory 11
Orientation and Navigation 11
Physiology of Migration 11
ADVANTAGES OF MIGRATION 13
STIMULUS FOR MIGRATION 15
WHEN BIRDS MIGRATE 17
Time of Year 17
Time of Day 20
SPEED OF FLIGHT AND MIGRATION 25
ALTITUDE OF FLIGHT AND MIGRATION 32
SEGREGATION DURING MIGRATION 35
By Individuals or Groups of Species 35
By Age 36
By Sex 38
By Kinds of Flocks 40
WHERE BIRDS MIGRATE 41
Migration by Populations Within Species 41
Fall Flights Not Far South of Breeding Range 42
Long Distance Migration 44
ORIENTATION AND NAVIGATION 47
INFLUENCE OF WEATHER 51
INFLUENCE OF TOPOGRAPHY 56
PERILS OF MIGRATION 58
Storms 58
Aerial Obstructions 58
Exhaustion 59
ROUTES OF MIGRATION 61
General Considerations 61
Flyways and Corridors 62
Narrow Routes 65
Converging Routes 65
Principal Routes From North America 69
Atlantic Oceanic Route 69
Atlantic Coast Route and Tributaries 70
Mackenzie Valley-Great Lakes-Mississippi
Valley Route and Tributaries 73
Great Plains-Rocky Mountain Routes 75
Pacific Coast Route 76
Pacific Oceanic Route 80
Arctic Routes 80
PATTERNS OF MIGRATION 82
Loops 82
Dog-legs 87
Pelagic Wandering 90
Leap-frogging 90
Vertical Migration 91
Pre-migratory Movements 91
Vagrant Migration 92
ORIGIN AND EVOLUTION OF MIGRATION 95
WHERE WE STAND 100
BIBLIOGRAPHY 102
LIST OF BIRD SPECIES MENTIONED IN TEXT 115
=PREFACE=
Frederick C. Lincoln's classic work on the "Migration of Birds"
first appeared in 1935. It was revised in 1950 and has been out of
print for several years, after selling over 140,000 copies. Unfilled
requests by many individuals, clubs, and institutions prompted the
Office of Conservation Education (now the Office of Public Affairs)
in the U.S. Fish and Wildlife Service to petition another update
for reissue. This publication incorporates the results gathered by
research biologists in the U.S. Fish and Wildlife Service to meet
these requests.
Lincoln's original intent was to present to the American public a
summary of the facts on bird migration as they existed in the early
1930's. He wrote with a style that made the topic fascinating to the
young and old, to the educated and uninformed, and to the ardent
observer as well as the backyard watcher. An attempt has been made
to retain this style, while incorporating material from often highly
technical research efforts. Much of the content and organization
of the original publication has been maintained, but new sections
were added to incorporate recent concepts and techniques. Other
concepts, known to be inconsistent with present knowledge, have been
deleted. Because graphics are of utmost importance in this type of
publication, most of the original figures were preserved and, where
appropriate, new illustrations have been added.
Since the previous edition, tremendous progress has been made in
researching and understanding bird migration; along with this
increased effort has come a substantial increase in the literature
devoted to the subject. Emphasis was given to reviewing literature
pertaining to migration studies conducted in North America after
1950, but a number of examples from the European literature have
been included to emphasize similarities and differences in migration
throughout the world. Because extensive author citations tend to
disrupt the flow of thought, they were kept to a minimum in the text.
However, this publication is essentially a review of the literature
on the subject as it existed in the early 1970's, and a rather
extensive bibliography has been included to cover all the papers
quoted in the text as well as the many used but not specifically
cited. The bibliography, then, is primarily intended for those
interested in pursuing the subject further.
=INTRODUCTION=
The changing picture of bird populations throughout the year
intrigues those who are observant and who wish to know the source and
destination of these birds. Birds are the most mobile creatures on
Earth. Even man with his many vehicles of locomotion does not equal
some birds in mobility. No human population moves each year as far
as from the Arctic to the Antarctic and return. Yet the Arctic terns
do--and without the aid of aircraft or compass.
Birds are adapted in their body structure, as no other creatures, to
life in the air. Their wings, tails, hollow bones, and internal air
sacs all contribute to this great faculty. These adaptations make it
possible for birds to seek out environments most favorable to their
needs at different times of the year. This results in the marvelous
phenomenon we know as migration--the regular, seasonal movement of
entire populations of birds from one geographic location to another.
Throughout the ages, migratory birds have been important as a
source of food after a lean winter and as the harbinger of a change
in season. The arrival of certain species has been heralded with
appropriate ceremonies in many lands; among the Eskimos and other
tribes, the phenomenon to this day is the accepted sign of the
imminence of spring, of warmer weather, and a change from winter
food shortages. The pioneer fur traders in Alaska and Canada offered
rewards to the Indian or Eskimo who saw the first flight of geese in
the spring, and all joined in jubilant welcome to the newcomers.
As the North American Continent became more thickly settled, the
large flocks of ducks and geese, rails, doves, and woodcock that
always had been hunted for food became objects of the enthusiastic
attention of an increasing army of sportsmen. Most of the nongame
species were found to be valuable also as allies of the farmer in his
never-ending warfare against insect pests. All species have been of
ever-increasing recreational and esthetic value for untold numbers of
people who enjoy watching birds. We began to realize our migratory
bird resource was an international legacy (that cannot be managed
alone by one state or country) and all nations were responsible for
its well-being. The need for laws protecting game and nongame birds,
as well as the necessity to regulate the hunting of diminishing game
species, followed as a natural course. In the management of this
wildlife resource, it has become obvious that continuous studies must
be made of the species' habits, environmental needs, and travels.
In the United States, the Department of the Interior recognizes
the value of this resource and is devoted to programs that will
ensure its preservation and wise use. Hence bird investigations are
made by the U.S. Fish and Wildlife Service, an arm of the Interior
Department, charged by Congress under the Migratory Bird Treaty
Act, with the duty of protecting those species that in their yearly
journeys, pass back and forth between the United States and other
countries.
For more than three-quarters of a century the Fish and Wildlife
Service and its predecessor, the Biological Survey, have been
collecting data on the important details of bird migration.
Scientists have gathered information concerning the distribution and
seasonal movements of many species throughout the New World, from the
Canadian archipelago south to the Argentine pampas. Supplementing
these investigations is the work of hundreds of U.S. and Canadian
university personnel and volunteer birdwatchers, who report on the
migrations and status of birds as observed in their respective
localities; while others place numbered bands on the legs of birds
to determine their movements from one place to another. These data,
stored in field notes, computer cards, scientific journals, and
on magnetic tape constitute an enormous reservoir of information
pertaining to the distribution and movements of North American birds.
It is the purpose of this publication to summarize these data and
present the more important facts about that little understood but
universally fascinating subject of bird migration. The U.S. Fish and
Wildlife Service is grateful to the many persons who have contributed
their knowledge so that other people, be they bird study classes,
conservation organizations, or just individuals interested in the
welfare of the birds, may understand and enjoy this precious resource
as well as preserve it for generations to come.
=THE HISTORY AND SCOPE OF MIGRATION=
The migrations of birds were probably among the first natural
phenomena to attract the attention and arouse the imagination of man.
Recorded observations on the subject date back nearly 3,000 years,
to the times of Hesiod, Homer, Herodotus, Aristotle, and others. In
the Bible there are several references to the periodic movements of
birds, as in the Book of Job (39:26), where the inquiry is made:
"Doth the hawk fly by Thy wisdom and stretch her wings toward the
south?" The author of Jeremiah (8:7) wrote: "The stork in the heavens
knoweth her appointed time; and the turtledove, and the crane, and
the swallow, observe the time of their coming." The flight of quail
that saved the Israelites from starvation in their wanderings through
the Sinai wilderness is now recognized as a vast migration between
their breeding grounds in eastern Europe and western Asia and their
winter home in Africa.
Of observers whose writings are extant, Aristotle, naturalist and
philosopher of ancient Greece, was one of the first to discuss
the subject of bird migration. He noted cranes traveled from the
steppes of Scythia to the marshes at the headwaters of the Nile, and
pelicans, geese, swans, rails, doves, and many other birds likewise
passed to warmer regions to spend the winter. In the earliest years
of the Christian era, Pliny the Elder, Roman naturalist, in his
"Historia Naturalis," repeated much of what Aristotle had said on
migration and added comments of his own concerning the movements of
starlings, thrushes, and European blackbirds.
Aristotle also must be credited with the origin of some superstitious
beliefs that persisted for several centuries. One of these, that
birds hibernated, became so firmly rooted, Dr. Elliott Coues
(1878),[1] an eminent American ornithologist, listed the titles of
no less than 182 papers dealing with the hibernation of swallows.
In fact the hibernation theory survived for more than 2,000 years,
and it was not until early in the nineteenth century that its
acceptance as an explanation for the winter disappearance of birds
was almost completely abandoned. Even after this, a few credulous
persons suggested this idea as an explanation for the disappearance
of chimney swifts in the fall before bands from wintering swifts were
finally reported as taken by Indians in Peru (Coffey 1944).
[1] Publications referred to parenthetically by date are listed in
the Bibliography, p. 102.
The followers of Aristotle believed the disappearance of many species
of birds in the fall was accounted for by their passing into a
torpid state where they remained during the cold season, hidden in
hollow trees, caves, or in the mud of marshes. Aristotle ascribed
hibernation not only to swallows, but also to storks, kites, doves,
and others. Some early naturalists wrote fantastic accounts of the
flocks of swallows allegedly seen congregating in marshes until
their accumulated weight bent into the water the reeds on which they
clung and thus submerged the birds. It was even recorded that when
fishermen in northern waters drew up their nets they sometimes had a
mixed "catch" of fish and hibernating swallows. Clarke (1912) quotes
Olaus Magnus, Archbishop of Upsala, who in 1555 published a work
entitled "Historia de Gentibus Septentrionalis et Natura," wherein he
observed that if swallows so caught were taken into a warm room they
would soon begin to fly about but would live only a short time.
Although the idea of hibernation as a regular method of spending
the winter is no longer accepted for any species of bird, certain
hummingbirds, swifts, and poorwills have been known to go into an
extremely torpid condition in cold weather (Jaeger 1948, 1949). Thus
Aristotle was at least partially vindicated.
Aristotle also was the originator of the theory of transmutation,
or the seasonal change of one species into another. Frequently
one species would arrive from the north just as another species
departed for more southerly latitudes. From this he reasoned the two
different species were actually one and assumed different plumages to
correspond to the summer and winter seasons.
Probably the most remarkable theory advanced to account for migration
is contained in a pamphlet, "An Essay toward the Probable Solution
of this Question: Whence come the Stork and the Turtledove, the
Crane, and the Swallow, when they Know and Observe the Appointed Time
of their Coming," mentioned by Clarke (1912: v. 1, 9-11) published
in 1703. It is written "By a Person of Learning and Piety," whose
"probable solution" stated migratory birds flew to the moon and there
spent the winter. Astronauts have so far failed to verify this.
Some people, who easily accepted the migratory travels of larger
birds, were unable to understand how smaller species, some of
them notoriously poor fliers, could make similar journeys. They
accordingly conceived the idea that larger species (e.g., storks
and cranes) carried their smaller companions as living freight.
In some southern European countries, it is still believed these
broad-pinioned birds serve as aerial transports for hosts of small
birds that congregate upon the Mediterranean shore awaiting the
opportunity for passage to winter homes in Africa. Similar beliefs,
such as hummingbirds riding on the backs of geese, have been found
among some tribes of North American Indians.
Today we realize that birds do not migrate by "hitching" rides
with other birds and that the scope of the migration phenomenon is
worldwide, not simply limited to the United States, the Northern
Hemisphere, or the world's land masses. The migration heritage is
developed just as extensively in Old World warblers migrating to
and from Europe and Africa as in our wood warblers traveling from
Canada and the United States to South America and back. One of the
fundamental differences in migration patterns of the Northern and
Southern Hemispheres is that no land species nesting in the South
Temperate Zone migrates into the North Temperate Zone, but a few
seabirds, such as the sooty shearwater, Wilson's storm-petrel, and
others, migrate north across the Equator over the vast ocean expanses
after nesting in the South.
=TECHNIQUES FOR STUDYING MIGRATION=
Before we discuss the many intricacies of how, when, and where birds
migrate, one should have a general idea of how migration data are
collected and what methods are currently being used to increase
our knowledge. Since this publication first appeared in 1935, many
new procedures have been used in the study of bird migration. One
of these, radar, has been an invaluable adaptation of a technique
developed for a quite different, but related, purpose.
Direct Observation
The oldest, simplest, and most frequently used method of studying
migration is by direct observation. Size, color, song, and flight of
different species all aid the amateur as well as the professional
in determining when birds are migrating. Studies begun by Wells
W. Cooke and his collaborators (Cooke 1888-1915) and continued by
his successors in the U.S. Bureau of Biological Survey (later U.S.
Fish and Wildlife Service) were of particular importance in the
earlier years of these investigations in North America. Some of the
largest and most interesting routes and patterns were sorted out by
tediously compiling and comparing literally thousands of observations
on whether a species was or was not seen in a given locality at a
particular time of the year. More recently, "The Changing Seasons"
reports by many amateur bird observers in _Audubon Field Notes_ (now
_American Birds_) have been a most important source of information on
direct observation of migration. In the aggregate, direct observation
has contributed much to our knowledge of migration, but, as will be
pointed out in other sections, until a few years ago, observers were
not aware of some of the biases in this technique.
The "moon watch" is a modification of the direct observation method.
It has long been known that many species of birds migrate at night.
Until recently, it was not apparent just how important nocturnal
migration really is. Significant information has been derived from
watching, through telescopes, the passage of migrating birds across
the face of a full moon. Since the actual percent of the sky observed
by looking through a telescope at the moon is extremely small
(approximately one-hundred thousandth of the observable sky), the
volume of birds recorded is small. On a night of heavy migration,
about 30 birds per hour can be seen. The fact that any birds are
observed at all is testimony to the tremendous numbers passing
overhead. Large-scale, cooperative moon-watching studies have been
organized and interpreted by George H. Lowery, Jr. (1951; Lowery and
Newman 1966).
Another specialized direct observation approach which has yielded
important information on the spatial and altitudinal distribution of
night migrating birds has been the use of small aircraft equipped
with auxiliary landing lights (Bellrose 1971). Major disadvantages
of night observation are that species cannot be identified and
that birds continue to migrate without a full moon. However, these
techniques do give information on the nocturnal migration movements
that could not be obtained by other methods.
Aural
An adjunct to the previously described nocturnal observation methods,
which has potential for species identification, is the use of a
parabolic reflector with attached microphone to amplify call (chip)
notes (Ball 1952; Graber and Cochrane 1959). This device, when
equipped with a tape recorder, can record night migrants up to 11,000
feet on nights with or without a full moon. A primary disadvantage is
that one cannot tell the direction a bird is traveling and there is
considerable difficulty in identifying the chip notes made by night
migrants. In addition, the bird may not call when it is directly over
the reflector and consequently it would not be recorded. These calls
are quite different from the notes we hear given by familiar birds
during the daytime while they are scolding an intruder or advertising
their territory.
Preserved Specimens
Reference material consisting of preserved bird skins with data on
time and place of collection exist in many natural history museums.
The essential ingredient in studying migration by this method is
to have an adequate series of specimens taken during the breeding
season so differences in appearance between geographically separated
breeding populations of the same species can be determined. Such
properly identified breeding specimens may be used for comparison
with individuals collected during migration to associate them with
their breeding areas (Aldrich 1952; Aldrich, Duvall, and Geis 1958).
This supplies a convenient way of recognizing and referring to
individuals representative of known populations wherever they may be
encountered.
Marking
If birds can be captured, marked, and released unharmed, a great
deal of information can be learned about their movements. Many
different marking methods have been developed to identify particular
individuals when they are observed or recaptured at a later date. A
few of the general methods are summarized in this section.
-------------------------------------------------
Bands, Collars, Streamers
-------------------------------------------------
Since 1920, the marking of birds with numbered leg bands in North
America has been under the direction of the U.S. Fish and Wildlife
Service in cooperation with the Canadian Wildlife Service. Every
year professional biologists and voluntary cooperators, working
under permit, place bands on thousands of birds, game and nongame,
large and small, migratory and nonmigratory, with each band carrying
a serial number and the legend, NOTIFY FISH AND WILDLIFE SERVICE,
WASHINGTON, D.C., or on the smaller sizes, an abbreviation. When
a banded bird is reported from a second locality, a definite fact
relative to its movements becomes known, and a study of many such
cases develops more and more complete knowledge of the details of
migration.
The records of banded birds are also yielding other pertinent
information relative to their migrations such as arrival and
departure dates, the length of time different birds pause on their
migratory journeys to feed and rest, the relation between weather
conditions and starting times for migration, the rates of travel for
individual birds, the degree of regularity with which individual
birds return to the summer or winter quarters used in former
years, and many other details. Many banding stations are operated
systematically throughout the year and supply much information
concerning the movements of migratory birds that heretofore could
only be surmised. The most informative banding studies are those
where particular populations of birds are purposely banded to produce
certain types of information when they are recovered. Examples
of such planned banding are the extensive marking of specific
populations of ducks and geese on their breeding grounds by the U.S.
Fish and Wildlife Service and the Canadian Wildlife Service, as
well as in "Operation Recovery," the cooperative program of banding
small landbirds along the Atlantic Coast (Baird et al. 1958). When
these banded birds are recovered, information concerning movements
of specific populations or the vulnerability to hunting is gained.
Colored leg bands, neck collars, or streamers can be used to identify
populations or specific individuals, and birds marked with easily
observed tags can be studied without having to kill or recapture
individuals, thus making it a particularly useful technique.
We have learned about the migratory habits of some species through
banding, but the method does have shortcomings. If one wishes to
study the migration of a particular species through banding, the
band must be encountered again at some later date. If the species
is hunted, such as ducks or geese, the number of returns per 100
birds banded is considerably greater than if one must rely on a bird
being retrapped, found dead, etc. For example, in mallards banded
throughout North America the average number of bands returned the
first year is about 12 percent. In most species that are not hunted,
less than 1 percent of the bands are ever seen again.
In 1935, Lincoln commented that, with enough banding, some of the
winter ranges and migration routes of more poorly understood species
would become better known. A case in point is the chimney swift,
a common bird in the eastern United States. This is a nonhunted
species that winters in South America. Over 500,000 chimney swifts
have been banded, but only 21 have been recovered outside the United
States (13 from Peru, 1 from Haiti, and the rest from Mexico).
The conclusion is simply this: Whereas banding is very useful for
securing certain information, the volume of birds that need to be
banded to obtain a meaningful number of recoveries for determining
migratory pathways or unknown breeding or wintering areas may be
prohibitive. One problem in interpretation of all banding results is
the fact that recoveries often reflect the distribution of people
rather than migration pathways of the birds.
Other methods used to mark individuals in migration studies include
clipping the tip end off a feather (not a major flight feather) with
a fingernail clipper or touching the feather with colored paint or
dye. This marking technique is obviously good for only as long as the
bird retains the feather (usually less than one year), but allows
the investigator to recognize whether the bird has been handled
previously or not.
Radio Tracking
One of the most promising methods of tracking the movements of
individual birds in migration has been developed in recent years. It
is called radio tracking, or telemetry, and consists of attaching to
a migrating bird a small radio transmitter that gives off periodic
signals or "beeps". With a radio receiving set mounted on a truck or
airplane, it is possible to follow these radio signals and trace the
progress of the migrating bird. One of the most dramatic examples
of this technique was reported by Graber in 1965. He captured a
grey-cheeked thrush on the University of Illinois campus and attached
a 2.5-gram transmitter to it (a penny weighs 3 grams). The bird was
followed successfully for over 8 hours on a course straight across
Chicago and up Lake Michigan on a continuous flight of nearly 400
miles at an average speed of 50 mph (there was a 27 mph tail wind
aiding the bird). It is interesting to note that while the little
thrush flew up the middle of Lake Michigan, the pursuing aircraft
skirted the edge of the lake and terminated tracking at the northern
end after running low on fuel while the bird flew on. The limitations
of radio telemetry, of course, are the size of the transmitter that
can be placed on birds without interfering with flight and the
ability of the receiving vehicle to keep close enough to the flying
bird to detect the signals. Despite this difficulty there has been
considerable development in the technology, and encouraging results
to date give promise for the future, particularly when receivers can
be mounted on orbiting satellites (Graber 1965; Bray and Corner 1972;
Southern 1965).
Radar Observation
One of the developments of our modern age of electronics has been
the discovery that migrating birds show up on radar screens used in
monitoring aircraft. At first, the screen images caused by flying
birds were a mystery to radar operators, and they designated the dots
"angels." Later when their nature was understood, students of bird
migration seized on the unique opportunity to obtain information on
movements of birds over extensive areas (Sutter 1957; Drury 1960;
Lack 1963a, b; Bellrose 1967; Graber 1968; and Gauthreaux 1972a, b).
Three types of radar have been used for studying birds: 1) general
surveillance radar, similar to ones located at airports, that scans
a large area and indicates the general time and direction of broad
movements of birds; 2) a tracking radar that records the path of an
airplane (or bird) across the sky by "locking on" to a designated
"target" and continuously following only that object; and 3) a
Doppler radar similar to those operated by law enforcement agencies
for measuring the speed of a passing automobile. The latter radar set
is useful in determining the speed of flying birds.
The use of radar in migration studies has been invaluable in
determining direction of mass movement, dates and times of departure,
height of travel, and general volume, especially at night. One
interesting fact to come out of current radar work is the discovery
of relatively large movements of warblers and other land birds
migrating over the seas rather than along the coastlines and in
directions observers were completely unaware of a few years ago.
Laboratory
-------------------------------------------------
Orientation and Navigation
-------------------------------------------------
Studies on how migrating birds orient (travel in one compass
direction) or navigate (travel toward a specific goal) have received
increasing emphasis in the past 20 years. These studies have focused
on the ability of birds to orient themselves by the position of the
sun and stars. Outstanding in this facet of research have been the
works of Matthews (1951, 1955), Kramer (1952, 1959, and 1961), Sauer
and Sauer (1960), Mewaldt and Rose (1960), Sauer (1961), Hamilton
(1962a, b), Schmidt-Koenig (1963, 1964), and Emlen (1969). The
basic method used in the experiments is to observe the direction in
which confined birds attempt to move during the period of migratory
restlessness. The birds are not permitted to have any view of the
landscape but only the sky above them. In some cases the positions
of the celestial bodies are changed by the use of mirrors to see the
effect on the orientation of the experimental birds. In other cases
the experiments are performed in planetariums so positions of the
stars in the artificial heavens can be manipulated and the effect
observed.
-------------------------------------------------
Physiology of Migration
-------------------------------------------------
The physiological basis for bird migration has received considerable
attention, particularly the effects of seasonal increases and
decreases in daylight and the seasonal rhythms occurring within
animals and referred to as "biological clocks." Investigations
in this field include the pioneering work on the relationship of
photoperiod (daylength) to migration by Rowan (1925, 1926) and many
subsequent studies (Wolfson 1940, 1945; Marshall 1961; King, Barker
and Farner 1963; King and Earner 1963; King 1963; Farner 1955, 1960;
and Farner and Mewaldt 1953). These studies have become ever more
deeply involved in the intricate relationships between photoperiod,
endocrine interactions, gonad development, fat deposition, and
migratory unrest. They add to our knowledge of the mechanisms that
regulate the migratory behavior we observe.
=ADVANTAGES OF MIGRATION=
Why should a bird subject itself to the rigors of a long migratory
journey twice a year if it can find all the requirements suitable
for existence in one locality? It seems well to consider briefly
the ends that are served by this annual round trip between breeding
grounds and winter quarters. Obviously, the migratory habit enables
a species to enjoy the summer of northern latitudes and to avoid the
severity of winter. In other words, migration makes it possible for
some species to inhabit two different areas during the seasons when
each presents favorable conditions. If it was not advantageous to
make the trip twice a year, natural selection would have eliminated
the tendency, but bird migration has become the rule over much of the
world rather than the exception.
By withdrawing in the spring to regions uninhabitable earlier in the
year, migrant species are generally assured of adequate space and
ample food upon their arrival in the winter-freed North, and those
nonmigratory kinds, which stay behind to nest, are also assured of
ample space for these activities.
Every pair of birds requires a certain amount of territory for the
performance of its reproductive duties, the extent of which varies
greatly between different species. This territory must be large
enough to provide adequate food, not only for the parent birds but
also for the lusty appetites of their young. In the Arctic summer,
24 hours of daylight allow the young to feed or be fed almost
continuously and rapid growth is apparent. The short breeding season
in northern latitudes exposes the vulnerable young to predation for a
brief period and prevents a build up of predator populations.
It cannot be said that the winter or summer area of every species
is entirely unsuited to the requirements of all of its members at
other seasons, because some individuals pass the winter season in
areas that are frequented only in summer by other individuals of
their species. Such species may have extensive breeding ranges with
wide climatic variations so that some individuals may actually be
permanently resident in a region where others of their kind are
present only in winter. Also, some individual song sparrows and blue
jays, for example, have been known to change their migratory status
(e.g., a particular bird may migrate one year and not the next or
vice versa). Thus, different individuals or populations within these
species appear to have different tolerances for climatic conditions.
The tendency of some birds to move southward at the approach of
winter is not always due to seasonal low temperatures. Experiments
have demonstrated many of our summer insect feeders, when confined in
outdoor aviaries, comfortably withstand temperatures far below zero
as long as abundant food is provided. The main consideration then,
is depletion of the food supply, caused by either the disappearance
or hibernation of insects or the mantle of snow or ice that prevents
access to seeds and other food found on or close to the ground or
submerged in water. Also, shortened hours of daylight may restrict
the ability of birds to obtain sufficient food at a time when low
temperatures require increased energy to maintain body heat. It is
noteworthy that some of our smaller birds, such as the chickadees,
can withstand a cold winter because their food supplies are always
available above ground on trees. When there is a good supply of pine
and spruce seeds, red-breasted nuthatches and crossbills will remain
through the winter in Canadian woods, but when these birds appear
abundantly in winter at southern latitudes, it may be concluded there
is a shortage of these foods in the North.
=STIMULUS FOR MIGRATION=
Modern views based on studies of bird behavior and physiology
indicate migration is a regular, annually induced movement, modified
by local weather conditions, but largely independent of them.
Migration is a phenomenon far too regular to be created anew each
season merely under stress of circumstances, such as need for food;
and it begins before the necessity for a change in latitude becomes
at all pressing. Swallows, nighthawks, shorebirds, and others may
start their southward movement while the summer food supply in the
North is at peak abundance. American robins and bluebirds may leave
abundant food in the South and press northward when food supplies
there are almost entirely lacking and severe cold and storms are
likely to cause their wholesale destruction. Regularity of arrival
and departure is one of the most impressive features of migration,
and since birds travel in a rather strict accordance with the
calendar, we might ask: "What phenomena, other than the regular
changes in length of day, occur with sufficient precision to act as a
stimulus for migration?"
Experimental work has abundantly demonstrated the effect of increased
light upon the growth, flowering, and fruiting of plants. Similarly,
Rowan's (1925) experiments with slate-colored juncos and the work of
numerous subsequent investigators showed, at least in some temperate
zone species of migratory birds, increasing periods of daylight
triggered sex organs to develop, fat to be deposited, and migration
restlessness to begin (King and Farner, 1963). When these conditions
develop to a certain level, the bird enters a "disposition to
migrate" and takes off for its breeding or wintering grounds. There
is reason to believe certain weather conditions influence the actual
time of departure and especially the rate of progress to the breeding
area.
This explanation of the stimulus for migration may apply very broadly
to birds that winter in temperate parts of the world and nest in the
same hemisphere but fails in those birds wintering in the tropics,
where little change in length of day occurs and even decreases during
the spring in regions south of the Equator. It might be asked: "If
the lengthening day is the stimulating factor, why should our summer
birds, wintering in the tropics, ever start north?" In addition,
if daylength influences when birds are stimulated to migrate, why
should they not all leave the same locality at the same time? Or, if
weather controls the departure of birds from a given area, should not
all the migrants leave when conditions are optimal and refrain from
departing when conditions are not so? Actually, the conditions that
place a bird in a disposition to migrate are probably the result of a
combination of factors affecting different species differently. Thus
not all birds arrive at this condition at the same time.
It has been demonstrated experimentally that Andean sparrows,
resident in equatorial regions, come into breeding condition twice
annually entirely independent of changing light periods (Miller
1963); evidently the breeding cycle is controlled by periodic
internal stimuli. Probably northern migrants that winter in
equatorial regions and beyond have their migratory urges controlled
by similar rhythms or biological clocks. Also, no evidence suggests
that the southward migration of birds is controlled by changing
periods of light even among species such as white-crowned sparrows,
for which this is a controlling factor in the spring. The fall
stimulus is probably an innate cyclic occurrence brought on by a
biological mechanism of unknown nature (King, Barker, and Farner
1963).
It is pertinent to point out that the migratory instinct appears
to be more or less transitory and not persistent over an extended
period. Migratory birds may be delayed en route, either by natural
conditions such as unusually abundant food supplies or forcibly by
man. If detained until the end of the migratory season, migrants may
not attempt to finish the journey because they apparently lose the
migratory impulse. In the fall and early winter of 1929, abundant
food and open water caused an unusual number of mallards to arrest
their migration and remain in western Montana and northern Idaho.
Later, however, when a heavy snowfall with subzero temperatures
suddenly cut off the food supply, great numbers of the birds
subsequently starved to death; a flight of a few hours could have
carried them to a region of open water and abundant food.
=WHEN BIRDS MIGRATE=
One ordinarily thinks of the world of birds as sedentary during two
periods each year, at nesting time, and in winter. For individuals
this is obviously the case, but when the entire avifauna of North
America or the world is considered, it is found that at almost all
periods there are some latitudinal movements of birds. A few of these
movements reoccur year after year with calendar-like regularity. Each
species, or group of species, migrates at a particular time of the
year and some at a particular time of the day. In this section some
of the interesting differences will be discussed as to when birds
migrate.
Time of Year
Some species begin their fall migrations early in July, and in
other species distinct southward movements can be detected late
into the winter. While some migrants are still traveling south,
some early spring migrants can be observed returning north through
the same locality. For example, many shorebirds start south in the
early part of July, while the goshawks, snowy owls, redpolls, and
Bohemian waxwings do not leave the North until forced to do so by the
advent of severe winter weather or a lack of customary food. Thus
an observer in the northern part of the United States may record
an almost unbroken southward procession of birds from midsummer to
winter and note some of the returning migrants as early as the middle
of February. While on their way north, purple martins have been known
to arrive in Florida late in January, and, among late migrants, the
northern movement may continue well into June. In some species the
migration is so prolonged that the first arrivals in the southern
part of the breeding range will have performed their parental duties
and may actually start south while others of the species are still on
their way north.
A study of these facts indicates the existence of northern and
southern populations of the same species that have quite different
migration schedules. In fall, migratory populations that nest
farthest south migrate first to the winter range because they finish
nesting first. For example, the breeding range of the black-and-white
warbler covers much of the eastern United States and southern Canada
northwest through the prairies to Great Bear Lake in Canada (Fig. 1).
It spends the winter in southern Florida, the West Indies, southern
and eastern Mexico, Central America, and northwestern South America.
In the southern part of its breeding range, it nests in April,
but those summering in New Brunswick do not reach their nesting
grounds before the middle of May. (Lines that connect points where
birds arrive at the same line are called isochronal lines. Fig. 2)
Therefore, if 50 days are required to cross the breeding range, and
if 60 days are allowed for reproductive activities and molting, they
would not be ready to start southward before the middle of July.
Then with a return 50-day trip south, the earliest migrants from the
northern areas would reach the Gulf Coast in September. Since adults
and young have been observed on the northern coast of South America
by August 21, it is very likely that they must have come from the
southern part of the nesting area.
[Illustration: _Figure 1. Summer and winter homes of the
black-and-white warbler. A very slow migrant, these birds nesting in
the northern part of the country take 50 days to cross the breeding
range. The speed of migration is shown in Fig. 2._]
Many similar cases might be mentioned, such as the black-throated
blue warblers still observed in the mountains of Haiti during the
middle of May when others of this species are en route through North
Carolina to New England breeding grounds. Redstarts and yellow
warblers, evidently the more southern breeders, are seen returning
southward on the northern coast of South America just about the time
the earliest of those breeding in the North reach Florida on their
way to winter quarters. Examples of the Alaska race of the yellow
warbler have been collected in Mississippi, Florida, and the District
of Columbia as late as October.
[Illustration: _Figure 2. Isochronal migration lines of the
black-and-white warbler, showing a very slow and uniform migration.
The solid lines connect places at which these birds arrive at the
same time. These birds apparently advance only about 20 miles per day
in crossing the United States._]
Students of migration know that birds generally travel in waves,
the magnitude of which varies with populations, species, weather,
and time of year. Characteristically, one will observe a few early
individuals come into an area followed by a much larger volume of
migrants. This peak will then gradually taper off to a few lingering
stragglers. If we plot numbers observed against time, the rising and
receding curve takes the form of a bell. In the northern part of the
United States there are two general migration waves. The first one in
early spring consists of "hardy" birds including many of our common
seed eaters like the finches, sparrows, and others. The second wave
occurs about a month later and consists primarily of insect-eating
birds, such as flycatchers, vireos, warblers, and the like. Each of
these species in turn has its own "curve" of migration in the major
wave.
Time of Day
Because most birds appear to be creatures of daylight, it seems
remarkable that many should select the night for extended travel.
Among the many nocturnal migrants are the smaller birds such as
rails, flycatchers, orioles, most of the sparrows, the warblers,
vireos, thrushes, and shorebirds. It is common to find woods and
fields on one day almost barren of bird life and on the following day
filled with sparrows, warblers, and thrushes, which indicates the
arrival of migrants during the night. Waterfowl hunters sitting in
their "blinds" frequently observe the passage of flocks of ducks and
geese, but great numbers of these birds also pass through at night;
the calls of Canada geese or the conversational gabbling of a flock
of ducks are common night sounds in spring and fall in many parts of
the country. Observations made with telescopes focused on the full
moon have shown processions of birds, and one observer estimated
their passage over his area at the rate of 9,000 per hour. This gives
some indication of the numbers of birds in the air at night during
peaks of migration. At such times radar observations have shown that
nocturnal migration begins about an hour after sundown, reaches a
peak shortly before midnight, and then gradually tapers off until
daybreak. Unless special circuits are installed in radar sets, bird
echoes during peak migration periods may cover a radar screen.
It has been suggested that small birds migrate by night to avoid
their enemies. To a certain extent this may be true because the
group includes not only weak fliers, such as the rails, but also the
small song and insectivorous birds, such as wrens, small woodland
flycatchers, and other species that habitually live more or less in
concealment. These birds are probably much safer making their flights
under the protecting cloak of darkness. Nevertheless, it must be
remembered that night migrants include also the snipe, sandpipers,
and plovers. Most shorebirds are usually found in the open and
are among the more powerful fliers, as some of them make annual
migratory flights over 2,000 miles nonstop across the ocean.
Night travel is probably best for the majority of birds chiefly from
the standpoint of feeding. Digestion is very rapid in birds and yet
the stomach of one killed during the day almost always contains food.
To replace the energy required for long flight, it is essential that
either food be obtained at comparatively short intervals or stores of
fat be laid on prior to migration. If the smaller migrants were to
make protracted flights by day they would arrive at their destination
at nightfall almost exhausted, but since they are entirely daylight
feeders, they would be unable to obtain food until the following
morning. Unless reserve energy was carried in the form of fat, the
inability to feed would delay further flights and result in great
exhaustion or possibly even death should their evening arrival
coincide with cold or stormy weather. By traveling at night, they
can pause at daybreak and devote the entire period of daylight
to alternate feeding and resting. This schedule permits complete
recuperation and resumption of the journey on a subsequent evening
after sufficient energy has been restored.
The day migrants include, in addition to some of the ducks and geese,
the loons, cranes, gulls, pelicans, hawks, swallows, nighthawks,
and swifts. Soaring birds, including broad-winged hawks, storks,
and vultures, can only migrate during the day because their mode
of flight makes them dependent on up-drafts created by heat from
the sun for their long distance travels. On the other hand, swifts
and swallows feed entirely on diurnal flying insects. The circling
flocks are frequently seen in late summer feeding as they travel
while working gradually southward. Formerly, great flocks of
red-tailed, Swainson's, and rough-legged hawks could be seen wheeling
majestically across the sky in the Plains States. In the East, good
flights of broad-winged, Cooper's, and sharp-shinned hawks are still
often seen, particularly along the Appalachian ridges.
Because many species of wading and swimming birds are able to feed at
all hours, they migrate either by day or night and are not accustomed
to seek safety in concealment. Some diving birds, including ducks
that submerge when in danger, often travel over water by day and over
land at night. Strong fliers like the snow geese can make the entire
trip from their staging area in James Bay, Canada, to the wintering
grounds on the Louisiana Gulf coast in one continuous flight. These
birds are seldom shot by hunters en route between these two points
but are often observed, when migrating, by aircraft pilots. Graham
Cooch of the Canadian Wildlife Service tracked a flight of the blue
phase of this species in 1955. The birds left James Bay on October
17 and arrived on the Gulf coast 60 hours later after an apparent
continuous flight over the 1,700-mile route at an average speed of 28
miles per hour. Golden plovers, likewise, probably make the southward
flight from the Arctic to the South American coast in one giant leap.
Other Arctic species on their northward flight in the spring might
prefer to fly at night in lower altitudes, but must necessarily fly
during the day at higher altitudes because of the length of the
days. Many warblers that normally fly at night may find themselves
over water at daybreak and be forced to keep flying during the day
until landfall is made.
[Illustration: _Figure 3. Migration of the blackpoll warbler. As the
birds move northward, the isochronal lines become farther apart,
which indicates that the warblers move faster with the advance of
spring. From April 30 to May 1 the average speed is about 30 miles
per day, while from May 25 to May 30 it increases to more than 200
miles._]
An interesting comparison of the flights of day and night migrants
may be made through a consideration of the spring migrations of
the blackpoll warbler and the cliff swallow. Both spend the winter
as neighbors in South America, but when the impulse comes to start
northward toward their respective breeding grounds, the warblers
strike straight across the Caribbean Sea to Florida (Fig. 3), while
the swallows begin their journey by a westward flight of several
hundred miles to Panama (Fig. 4). From there they move leisurely
along the western shore of the Caribbean Sea to Mexico, and,
continuing to avoid a long trip over water, go completely around the
western end of the Gulf of Mexico. This circuitous route adds more
than 2,000 miles to the journey of the swallows that nest in Nova
Scotia. The question may be asked: "Why should the swallow select
a route so much longer and more roundabout than that taken by the
blackpoll warbler?" The explanation is simple. The swallow is a day
migrant while the warbler travels at night. The migration of the
warbler is made up of a series of long nocturnal flights alternated
with days of rest and feeding in favorable localities. The swallow,
on the other hand, starts its migration several weeks earlier and
catches each day's ration of flying insects during its aerial
evolutions, while slowly migrating. The 2,000 extra miles flown along
the insect-teeming shores of the Gulf of Mexico are exceeded by the
great distances covered by these birds in normal pursuit of food.
[Illustration: _Figure 4. Migration of the cliff swallow. A day
migrant that, instead of flying across the Caribbean Sea as does the
blackpoll warbler (see Fig. 3), follows the coast of Central America,
where food is readily obtained._]
Although most of our smaller birds make their longest flights at
night, close observation shows travel is continued to some extent
by day. During the latter half of a migratory season birds may show
evidence of an overpowering desire to hasten to their breeding
grounds. At this time flocks of birds maintain a movement in the
general direction of the seasonal journey while feeding on or near
the ground. Sometimes they travel hurriedly, and while their flights
may be short, they can cover an appreciable distance in the course of
a day.
=SPEED OF FLIGHT AND MIGRATION=
There is a widespread misconception among people concerning the speed
at which birds can fly. One often hears stories of birds flying "a
mile a minute." While undoubtedly some birds can and do attain this
speed, such cases are exceptional, and it is safe to say that, even
when pressed, few can develop an air speed of 60 miles per hour.
Birds generally have two greatly differing speeds, one being the
normal rate for ordinary purposes, and an accelerated speed for
escape or pursuit. All birds, except the heavy-bodied, small-winged
species such as auks, grebes, and other divers, have a reserve speed
that may be double the normal rate.
Although it was thought for a long time that migratory flights were
made at normal cruising speeds, Harrisson (1931) and Meinertzhagen
(1955) showed that migration speeds were in between cruising speeds
and escape speeds. The theory that migrating birds attain high speeds
received encouragement from the German ornithologist Gatke (1895)
who, for many years, observed birds at the island of Heligoland. He
postulated that the bluethroat, a species of thrush smaller than the
American hermit thrush, could leave African winter quarters at dusk
and reach Heligoland at dawn; this flight would mean a sustained
speed of 200 miles per hour! He also thought the American golden
plover flew from the coast of Labrador to Brazil in 15 hours at the
tremendous speed of 250 miles per hour. Most ornithologists now
consider these conclusions to be unwarranted.
Reliable data on the speed of birds are accumulating slowly. Accurate
measurements are difficult to obtain unless the bird travels over a
measured course and wind conditions at the level of flight are known.
Several subtle factors, besides wind and pursuit, can influence the
speed of a flying bird. For instance, species that have a courtship
flight often reach their maximum speeds then. Small woodland birds
often fly faster across an open area where they might be attacked by
a bird of prey than under cover where there is less danger. Birds
in flocks generally fly faster than when flying alone. A thermal
draft may induce an almost imperceptible air movement at the Earth's
surface, but a good glider with motionless wings may make 35 miles
per hour on a current of air that is rising vertically at less than
2 miles per hour. If the bird coasts downhill at a slight angle in
still air, it can attain a similar speed.
For sustained flight, it may be generally concluded that larger birds
fly faster than smaller birds. A common flying speed of ducks and
geese is between 40 and 50 miles per hour, but among the smaller
birds it is much less. Herons, hawks, horned larks, ravens, and
shrikes, timed with the speedometer of an automobile, have been found
to fly 22 to 28 miles per hour, whereas some of the flycatchers fly
at only 10 to 17 miles per hour. Even such fast-flying birds as the
mourning dove rarely exceed 35 miles per hour. A peregrine falcon
will have difficulty catching a pigeon during a level chase at 60
miles per hour, but this predator can probably exceed 200 miles per
hour during a swoop from a greater height onto its prey.
The speed of migration is quite different from that attained in
forced flights for short distances. A sustained flight of 10 hours
per day would carry herons, hawks, crows, and smaller birds from
100 to 250 miles, while ducks and geese might travel as much as 400
to 500 miles in the same period (without the aid of a tail wind).
Measured as straight line distances, these journeys are impressive
and indicate birds could travel from the northern United States or
even from northern Canada to winter quarters in the West Indies,
Central, or South America in a relatively short time. It is probable
that individual birds do make flights of the length indicated and
that barn swallows seen in May on Beata Island, off the southern
coast of the Dominican Republic, may have reached that point after a
nonstop flight of 350 miles across the Caribbean Sea from the coast
of Venezuela.
Radar has given us some of our best estimates of ground speeds for
migrating flocks, especially at night. Radar echoes, identified
as shorebirds migrating off the New England coast, moved steadily
about 45 miles per hour for several hours; songbird echoes typically
traveled around 30 miles per hour (Drury 1960). Some birds appear
to reduce flight speed in proportion to the degree of assistance or
resistance. The literature is in some disagreement on the flight
speed of birds and the influence of wind, but good radar observations
coupled with accurate measurements of winds aloft will help give us
a more accurate estimate of migrating speeds for different species
under varying wind conditions.
The intensity of migration depends on circumstances including the
need for haste. In fall the flights are more likely to be performed
in a leisurely manner, so that after a flight of a few hours
the birds often pause to feed and rest for one or several days,
particularly if they find themselves in congenial surroundings.
Some indication of this is found in the recoveries of banded birds,
particularly waterfowl. If we consider only the shortest intervals
between banding in the North and subsequent recovery in the South, it
is found that usually a month or more is taken to cover straight-line
distance of a thousand miles. For example, a black duck banded
at Lake Scugog, Ontario, was killed 12 days later at Vicksburg,
Mississippi. If the bird was taken shortly after its arrival, the
record would indicate an average daily flight of 83 miles, a distance
that could have been covered in about 2 hours' flying time. Among
the thousands of banding records of ducks and geese, evidences of
rapid migrations are decidedly scarce, for with few exceptions, all
thousand-mile flights have required 2 to 4 weeks or more. Among
sportsmen, the blue-winged teal is well known as a fast-flying
duck and quite a few of these banded on Canadian breeding grounds
have covered 2,300 to 3,000 miles in a 30-day period. Nevertheless,
the majority of those that have traveled to South America were not
recovered in that region until 2 or 3 months after they were banded.
Probably the fastest flight over a long distance for one of these
little ducks was one made by a young male that traveled 3,800 miles
from the delta of the Athabaska River, northern Alberta, Canada,
to Maracaibo, Venezuela, in exactly 1 month. This flight was at an
average speed of 125 miles per day. A very rapid migration speed was
maintained by a lesser yellowlegs banded at North Eastham, Cape Cod,
Massachusetts, on 28 August 1935 and killed 6 days later, 1,900 miles
away, at Lamentin, Martinique, French West Indies. This bird traveled
an average daily distance of more than 316 miles.
It seems probable that most migratory journeys are performed at
little more than the normal, unforced rate of flight, as this would
best conserve the strength of the birds. Migrating birds passing
lightships and lighthouses or crossing the face of the moon have been
observed to fly without hurry or evidence of straining to attain high
speed. The speed or rate of migration would therefore depend chiefly
on the duration of flights and tail wind velocity.
The speed of migration is demonstrated by the dates of arrival,
particularly during the spring movement. The Canada goose affords a
typical example of regular but slow migration. Its advance northward
is at the same rate as the advance of the season (Fig. 5). In fact,
the isotherm of 35° F appears to be a governing factor in the speed
at which these geese move north. (An isotherm is a line that connects
points that have the same temperature at the same time.) From an
evolutionary viewpoint we might expect this. If the geese continually
advanced ahead of the 32° F isotherm, they would always find food
and water frozen and unavailable. By migrating north just behind the
advance of this isotherm, birds that breed in the far north will find
food and open water available and have as long a breeding season as
the climate will allow.
Few species perform such leisurely migrations; many wait in their
winter homes until spring is well advanced, then move rapidly to
their breeding grounds. Sometimes this advance is so rapid, late
migrants actually catch up with species that may have been pressing
slowly but steadily northward for a month or more. The following
several examples of well-known migrants illustrate this.
The grey-cheeked thrush, which winters in the
Colombia-Ecuador-Peru-Venezuela-British Guiana area, does not start
its northward journey until many other species are well on their way.
It does not appear in the United States until the last of April--25
April near the mouth of the Mississippi and 30 April in northern
Florida (Fig. 6). A month later, or by the last week in May, the bird
is seen in northwestern Alaska. Therefore, the 4,000-mile trip from
Louisiana was made at an average distance of about 130 miles per day.
[Illustration: _Figure 5. Migration of the Canada goose. The
northward movement keeps pace with the progress of spring, because
the advance of the isotherm of 35 F agrees with that of the birds._]
[Illustration: _Figure 6. Isochronal migration lines of the
gray-cheeked thrush, an example of rapid migration. The distance
from Louisiana to Alaska is about 4,000 miles and is covered at
an average speed of about 130 miles per day. The last part of the
journey is covered at a speed several times what it is in the
Mississippi Valley._]
Another example of rapid migration is furnished by the yellow
warbler. This species winters in the Tropics and reaches New Orleans
about April 5, when the average temperature is 65° F. By traveling
north much faster than the spring season progresses, this warbler
reaches its breeding grounds in Manitoba the latter part of May, when
the average temperature is only 47° F. They encounter progressively
colder weather over their entire route and cross a strip of country
in the 15 days from May 11 to 25 that spring temperatures normally
take 35 days to cross. This "catching up" with spring is habitual in
species that winter south of the United States as well as in most
northern species that winter in the Gulf States. There appears to be
only six exceptions to this rule: the Canada goose, the mallard, the
pintail, the common crow, the red-winged blackbird, and the robin.
The snow goose presents a striking example of a late but very rapid
spring migration. Most all of these geese winter in the great coastal
marshes of Louisiana, where every year over 400,000 spend the
winter and congregations of 50,000 or more may be seen grazing in
the "pastures" or flying overhead in flocks of various sizes. Their
breeding grounds are chiefly on Baffin and Southampton Islands in the
northern part of Hudson Bay where conditions of severe cold prevail
except for a few weeks each year. The birds are not stimulated to
migrate even though the season in their winter quarters is advancing
rapidly while their nesting grounds are still covered with a heavy
blanket of ice and snow. This suggests the stimulus for spring
departure is regulated by an internal mechanism, such as development
of the gonads. Accordingly, blue geese remain in the coastal marshes
until the last of March or the first of April, when the local birds
are already busily engaged in reproduction. The flight northward
is rapid, almost nonstop so far as the United States is concerned;
although the birds are sometimes recorded in large numbers in the
Mississippi Valley, eastern South Dakota, and southeastern Manitoba,
there are few records anywhere along the route of the great flocks
that winter in Louisiana. When the birds arrive in the James Bay
region, they apparently enjoy a prolonged period of rest because they
are not seen in the vicinity of their breeding grounds until the
first of June. During the first 2 weeks of that month, they pour onto
the Arctic tundra by the thousands, and each pair immediately sets
about the business of rearing a brood.
The American robin has been mentioned as a slow migrant, and, as a
species, it takes 78 days to make the 3,000-mile trip from Iowa to
Alaska, a stretch of country that is crossed by advancing spring in
68 days. In this case, however, it does not necessarily mean that
individual robins are slow. The northward movement of the species
probably depends upon the continual advance of birds from the rear,
so that the first individuals arriving in a suitable locality are
the ones that nest in that area, while the northward movement of the
species is continued by those still to come.
There is great variation in the speed of migration at different
latitudes of the broad region between the Gulf of Mexico and the
Arctic Ocean. The blackpoll warbler again furnishes an excellent
example (Fig. 3). This species winters in northwestern South
America and starts to migrate north in April. When the birds reach
the southern United States, some individuals fly northwest to the
Mississippi Valley, north to Manitoba, northwest to the Mackenzie
River, and then almost due west to western Alaska. A fairly uniform
average distance of 30 to 35 miles per day is maintained from the
Gulf to Minnesota, but a week later this species has reached the
central part of the Mackenzie Valley, and by the following week it
is observed in northwestern Alaska. During the latter part of the
journey, therefore, many individuals must average more than 200 miles
per day. Thirty days are spent traveling from Florida to southern
Minnesota, a distance of about 1,000 miles, but scarcely half that
time is used to cover the remaining 2,500 miles to Alaska. Increased
speed across western Canada to Alaska is also shown by many other
birds (Figs. 2, 4, 6). A study of all species traveling up the
Mississippi Valley indicates an average speed of about 23 miles
per day. From southern Minnesota to southern Manitoba 16 species
maintain an average speed of about 40 miles per day. From that point
to Lake Athabaska, 12 species travel at an average speed of 72 miles
per day, while 5 others travel to Great Slave Lake at 116 miles per
day, and another 5 species cover 150 miles per day to reach Alaska.
This change is in correlation with a corresponding variation in the
isothermal lines, which turn northwestward west of the Great Lakes.
As has been previously indicated, the advance of spring in the
northern interior is much more rapid than in the Mississippi Valley
and on the Gulf coast. In other words, in the North spring comes with
a rush, and, during the height of migration season in Saskatchewan,
the temperature in the southern part of the Mackenzie Valley just
about equals that in the Lake Superior area, 700 miles farther south.
Such conditions, coupled with the diagonal course of the birds
across this region of fast-moving spring, exert a great influence
on migration and are probably factors in the acceleration of travel
speed. However, it should be remembered that the birds are getting
closer to the breeding season and may be stimulated to travel faster
for this reason.
Thus it has been shown that the rate of migration varies greatly
under varying circumstances. Radar investigations along the eastern
coasts of the United States and England indicate spring migration is
several miles per hour faster than in the fall. Also, directions of
migrations in spring were much less diverse than in the fall, which
suggests less time lost in passage (Tedd and Lack 1958; Nisbet and
Drury 1967a). King and Farner (1963) found the same species put on
more fat preparatory to migration in the spring. This would give the
migrants greater energy reserves for longer flights at that season.
=ALTITUDE OF FLIGHT AND MIGRATION=
The factors regulating the heights of bird migration are not clear.
High-altitude flight may be used to locate familiar landmarks, fly
over fog or clouds, surmount physical barriers, gain advantage
of a following wind, or maintain a better physiological balance.
Meteorological conditions probably account for most of the
high-altitude records. Wind conditions at ground level are usually
quite different in direction and velocity than at points higher up.
In general, human estimates of bird heights are quite unreliable
except under special conditions, and these estimates will vary with
the eyesight of the observer. Lucanus (1911) found a European sparrow
hawk could be distinguished at 800 feet but disappeared from sight at
2,800 feet. A rook (a European member of the crow family) could be
recognized at 1,000 feet but disappeared from sight at 3,300 feet.
Meinertzhagen (1955) did an interesting experiment with an inflated
model of a vulture painted black; it had a wing expanse of 7 feet 10
inches. When released from an airplane at 4,700 feet, it was barely
visible and invisible without binoculars at 5,800 feet. At 7,000 feet
it was not picked up even when ×12 binoculars were used.
At one time students of bird migration believed normal migratory
movements took place at heights above 15,000 feet. They reasoned,
somewhat uncertainly, that flying became easier as altitude was
gained. It has now been shown, through comprehensive radar studies,
that 95 percent of the migratory movements occur at less than 10,000
feet, and the bulk of the movements occur under 3,000 feet. However,
birds can and do fly well over 15,000 feet without apparent ill
effects. The physiology of long-distance flight at high altitudes
is of great interest but can only be touched on briefly in this
discussion.
Bird flight at 20,000 feet, where less than half the oxygen is
present than at sea level, is impressive if only because the work
is achieved by living muscle tissue. A Himalayan mountain climber
at 16,000 feet was rather amazed when a flock of geese flew north 2
miles over his head honking as they went (Swan 1970). At 20,000 feet
a man has a hard time talking and running or other rapid movements
are out of the question; but those geese were probably flying at
27,000 feet and even calling while they traveled at this tremendous
height.
Accurate observations on the altitude of migratory flights is
scanty, although altimeter observations from airplanes and radar are
becoming more frequent in the literature. An example is the report
of a mallard struck by a commercial airliner at 21,000 feet over
the Nevada desert (Manville 1963). It is, of course, obvious that
some birds must cross mountain ranges during migration and attain
great altitudes. Numerous observations have come from the Himalayas
(Geroudet 1954; Swan 1970). Observers at 14,000 feet recorded storks
and cranes flying so high that they could be seen only through field
glasses. In the same area large vultures were seen soaring at 25,000
feet and an eagle carcass was found at 26,000 feet. The expedition to
Mt. Everest in 1952 found skeletons of a pintail and a black-tailed
godwit at 16,400 feet on Khumbu Glacier (Geroudet 1954). Bar-headed
geese have been observed flying over the highest peaks (29,000+
feet) even though a 10,000-foot pass was nearby. Probably 30 or more
species regularly cross these high passes (Swan 1970).
Except to fly over high mountain ranges, birds rarely fly as high as
those traveling down the western Atlantic (Richardson 1972). Many
of these birds are making long-distance flights to eastern South
America and beyond. Therefore, flight at high altitudes in this
region is probably advantageous for them. Richardson postulated
stronger advantageous tail winds were found higher up and the cooler
air minimized evaporative water losses. This investigator found
air temperatures averaged 35° F at 10,000 feet over Nova Scotia in
September. The lower the ambient temperature, the more heat can be
lost by convection and the less water is required for cooling. Also,
a bird flying high can achieve the same range as one flying at sea
level but must cruise at a higher speed with a corresponding increase
in power output and oxygen consumption. But the increased cruising
speed results in shorter flight time and less interference from wind
(Pennycuick 1969).
Another postulate favoring the high-altitude flying theory was
that the wonderful vision of birds was their sole guidance during
migratory flights. To keep landmarks in view, birds were obliged
to fly high, particularly when crossing wide areas of water. This
will be considered in greater detail in the section, "Orientation
and Navigation," so here it will be sufficient to say that birds
rely only in part upon landmarks to guide them on migration. Also,
it must be remembered that definite physical limitations to the
range of visibility exist even under perfect atmospheric conditions.
Chief of these is the curvature of the earth's surface. Thus, if
birds crossing the Gulf of Mexico to Louisiana and Florida flew at a
height of 5 miles, they would still be unable to see a third of the
way across (during daylight hours). And yet this trip is made twice
each year, much of the distance probably at night, by thousands of
thrushes, warblers, and others.
The altitude of migration depends upon the species of bird, weather,
time of day or year, and geographical features. Nocturnal migrants,
studied by radar, appear to fly at different altitudes at different
times during the night. Birds generally take off shortly after
sundown and rapidly gain maximum altitude. This peak is maintained
until around midnight, then the travelers gradually descend until
daylight. For most small birds the favored altitude appears to
be between 500 and 1,000 feet (Bellrose 1971), but radar studies
have found some nocturnal migrants (probably shorebirds) over the
ocean were at 15,000 or even 20,000 feet (Lack 1960b; Nisbet 1963b;
Richardson 1972). Observations made from lighthouses and other
vantage points indicate that certain migrants commonly travel at
altitudes of a very few feet to a few hundred feet above sea or land.
Sandpipers, northern phalaropes, and various sea ducks have been
seen flying so low they were visible only as they topped a wave.
Observers stationed at lighthouses and lightships off the English
coast have similarly recorded the passage of landbirds flying just
above the surface of the water and rarely above 200 feet. During the
World Wars, broad areas in the air were under constant surveillance,
and many airplane pilots and observers took more than a casual
interest in birds. Of the several hundred records resulting from
their observations, only 36 were of birds flying above 5,000 feet and
only 7 above 8,500 feet. Cranes were once recorded at an altitude
of 15,000 feet, while the lapwing was the bird most frequently seen
at high levels, 8,500 feet being its greatest recorded altitude.
Records of the U.S. Civil Aeronautics Administration show that over
two-thirds of all the bird-aircraft collisions occur below 2,000 feet
and practically none occur above 6,000 feet (Williams 1950).
Recently, radar has aided greatly in determining differences in the
altitude of bird flight. Nocturnal migrants appear to fly slightly
higher, on the average, than diurnal migrants, but daytime flights
may be influenced more by cloud cover (Lack 1960a; Eastwood and Rider
1965). Bellrose (1971) found little difference in the altitudinal
distribution of small nocturnal migrants under clear or overcast
skies. Many night migrating birds are killed each year by striking
lighthouses, television towers or other man-made illuminated
obstructions, but this does not furnish proof that low altitudes are
generally used during nocturnal flight because these accidents occur
chiefly in foggy weather. Under such conditions, migrating birds seem
to be attracted to and confused by lights. Seabirds, such as loons,
eiders, and scoters, generally fly very low over the water but gain
altitude when land is crossed. The reverse is true for landbirds
(Dorst 1963; Bergman and Donner 1964; Eastwood and Rider 1965).
There may be a seasonal difference in the altitude of migration,
but the evidence is conflicting. Radar echoes studied by Bellrose
and Graber in Illinois (1963) showed fall migrants flew higher than
spring migrants. They speculated this difference was related to the
winds during the fall being more favorable for southerly migration
at higher altitudes, while winds at these altitudes in the spring
would be less favorable for northerly migration. Eastwood and Rider
(1965) studied seasonal migration patterns in England and found the
reverse to be true. They suggested one reason for this seasonal
difference was that flocks of fall migrants included many young birds
whose flight capabilities are inferior to adults and consequently are
unable to achieve the higher altitudes in the fall.
=SEGREGATION DURING MIGRATION=
By Individuals or Groups of Species
During the height of northward movement in spring, the woods and
thickets may suddenly be filled with several species of wood
warblers, thrushes, sparrows, flycatchers, and other birds.
It is natural to conclude they traveled together and arrived
simultaneously. Probably they did, but such combined migration is by
no means the rule for all species.
As a group, the wood warblers probably travel more in mixed companies
than do any other single family of North American birds. In spring
and fall, the flocks are likely to be made up of the adults and young
of several species. Sometimes swallows, sparrows, blackbirds, and
some of the shorebirds also migrate in mixed flocks. In the fall,
great flocks of blackbirds frequently sweep south across the Plains
States, with common grackles, red-winged blackbirds, yellow-headed
blackbirds, and Brewer's blackbirds included in the same flock.
On the other hand many species keep strictly to themselves. It would
be difficult for any other kind of bird to keep company with the
rapid movements of the chimney swift. Besides flight speed, feeding
habits or roosting preferences can be so individual as to make
traveling with other species incompatible. Nighthawks also fly in
separate companies, as do crows, waxwings, crossbills, bobolinks, and
kingbirds. Occasionally, a flock of ducks will be observed to contain
several species, but generally when they are actually migrating,
individuals of each species separate and travel with others of their
own kind.
Although different species generally do not migrate together, we
often find many species passing through an area at the same time.
If the different kinds of birds observed in a specific area are
counted every day throughout the entire migration season, this count
often rises and falls much like the bell-shaped curve exhibited when
the number of individuals of a given species are counted through
the same time period. Figure 7 shows two peaks in the number of
species passing through the desert at the north end of the Gulf of
Eilat (=Akaba) in the Red Sea. These two peaks happen to coincide
with peaks in the numbers of individuals (mostly from the order of
perching birds) traveling through the area. Therefore, in the latter
part of March and again in April, one notices not only more birds in
the area but also more different kinds.
Closely related species or species that eat the same food organisms
are not often found migrating through the same area at the same time.
Ornithologists call this species replacement. In North America,
peaks in the migration of the five kinds of spotted thrushes
generally do not coincide. Dates of departure in these species have
evolved so all the individuals of these closely related birds do
not converge on one area at the same time and subsequently exhaust
the food supply. By selection of staggered peak migration dates,
evolution has distributed the members of this family more or less
evenly throughout the entire season. Likewise, in the eastern
Mediterranean area, we find a similar situation in spring migration
for three closely related buntings; Cretzschmar's bunting comes
through first, followed a few weeks later by the Ortolan bunting and,
at the tail end of the migration period, the black-headed bunting
appears (Fig. 8).
By Age
The adults of most birds leave the young when they are grown. This
gives the parents an opportunity to rest and renew their plumage
before starting for winter quarters. The young are likely to move
south together ahead of their parents. This has been documented in a
number of species including our mourning dove, the common swift of
Europe, and storks. Mueller and Berger (1967) found an age-specific
migration pattern in sharp-shinned hawks passing through Wisconsin.
The immatures were much in evidence during mid-September while the
adults came through a month later. Far to the south in the Antarctic,
young Adelie penguins depart for northern wintering grounds much
earlier than adults.
[Illustration: _Figure 7. Average number of species captured daily
in mist nets during spring migration at Eilat, Israel, in 1968. The
number of species passing through an area on migration will rise and
fall similar to the number of birds counted in the area. In this case
two major movements came through about 1 month apart._]
In a few species, adults depart south before the young. Adult golden
plovers, Hudsonian godwits, and probably most of the Arctic breeding
shorebirds leave the young as soon as they are capable of caring for
themselves and set out for South America ahead of the juveniles.
Likewise, data for the least flycatcher indicate adults migrate
before the young, but Johnson (1963) did not find this segregation
in the Hammond's flycatcher. In Europe, adult red-backed shrikes are
known to migrate ahead of their young.
[Illustration: Cretzschmar's Bunting Ortolan Bunting Black-headed
Bunting]
[Illustration: _Figure 8. Average number of three species of buntings
captured daily in mist nets during spring migration at Eilat, Israel,
in 1968. Closely related species that migrate through the same area
often appear at different times. Thus species that may eat the same
foods do not compete with each other._]
In contrast to this loss of parental concern, geese, swans, and
cranes remain in family groups throughout migration. The parent
birds undergo a wing molt that renders them flightless during the
period of growth of their young so that both the adults and immatures
acquire their flight capabilities at the same time and are able to
start south together. Large flocks of Canada geese, for example, are
composed of many families banded together. When these flocks separate
into small V-shaped units it is probably correct to assume an old
goose or gander is leading the family. After female ducks start to
incubate their eggs, the males of most species of ducks flock by
themselves and remain together until fall. When segregation of the
sexes such as this occurs the young birds often accompany their
mothers south. Murray and Jehl (1964) concluded from mist-netting
many thousands of migrant passerines at Island Beach, New Jersey,
that adults and juveniles travel at approximately the same time.
By Sex
Males and females of some species may migrate either simultaneously
or separately. In the latter case it is usually the males, rarely the
females, that arrive first. Sometimes great flocks of male red-winged
blackbirds reach a locality several days before any females; this is
particularly the rule in spring. The first robins are usually found
to be males, as are also the first song sparrows, rose-breasted
grosbeaks, and scarlet tanagers. In Europe, the three buntings
mentioned previously are also segregated as to sex during migration.
Figure 8 shows two prominent peaks for both the Cretzschmar's and
Ortolan buntings; during passage the first peak was primarily males
while the second peak consisted mostly of females. This early arrival
of males on the breeding grounds is associated with territorial
possession whereby the male selects the area where it intends to
breed and each individual attempts to protect a definite territory
from trespass by other males of his own kind, while announcing his
presence to rival males and later arriving females by song or other
display. The female then selects the site where she wishes to nest.
The long-billed marsh wren is a noteworthy example; the males may
enthusiastically build several nests before the females arrive. In
the fall, common and king eiders are sexually segregated during
migration. During July, flocks crossing Point Barrow are composed
almost entirely of males, while after the middle of August the
flocks are almost all females (Thompson and Person 1963). In the
Chicago area, Annan (1962) reported that some males, such as the
hermit thrush, Swainson's thrush, gray-cheeked thrush, and veery,
arrive before any females and predominate during the first week of
occurrence.
In a few species the males and females apparently arrive at the
breeding grounds together and proceed at once to nest building. In
fact, among shorebirds, ducks, and geese, courtship and mating often
takes place in whole or in part while the birds are in the South or
on their way north, so that when they arrive at the northern nesting
grounds they are paired and ready to proceed at once with raising
their families. Mallards and black ducks may be observed in pairs
as early as December, the female leading and the male following
when they take flight. Naturally, these mated pairs migrate north
in company, and it was largely to protect such pairings that duck
shooting in spring was abolished by Federal law.
In the coastal subspecies of the western flycatcher, the sexes appear
to migrate in synchrony during the spring in contrast to migration
of Hammond's flycatcher in which the adult males usually precede the
females (Johnson 1973). Both sexes of the common blackcap of Europe
appear to migrate together at least across the eastern end of the
Mediterranean during the spring (Fig. 9).
[Illustration: Blackcap]
[Illustration: _Figure 9. Numbers of male and female blackcaps
captured daily in mist nets during spring migration at Eilat, Israel,
in 1968. At this point in their migration the sexes are passing
through the area at the same time. In other species (e.g., the
buntings in Fig. 8), the males often precede the females._]
By Kinds of Flocks
Migratory flights are frequently accomplished in close flock
formation, as with shorebirds, blackbirds, waxwings, and especially
some of the buntings, longspurs, juncos, and tree sparrows. Other
species maintain a very loose flock formation; examples are turkey
vultures, hawks, swifts, blue jays, swallows, warblers, and
bluebirds. Still others, the grebes, snowy owls, winter wrens,
shrikes, and belted kingfishers, ordinarily travel alone, and when
several are found in close proximity it is an indication they have
been drawn together by unusual conditions, such as abundant food.
Just as flocking among resident birds provides group protection
against predation, flocking in migration greatly facilitates the
attainment of destination (Pettingill 1970). The V-shaped flocks
often associated with Canada geese have a definite energy conserving
function by creating favorable air currents for every member of the
flock but the leader; when the leader becomes tired, it will often
change places with a member behind. Night migrating flocks generally
fly in looser formations than do day migrating flocks.
=WHERE BIRDS MIGRATE=
Migration by Populations Within Species
Both length and duration of migratory journeys vary greatly between
families, species, or populations within a species. Bobwhite, western
quails, cardinals, Carolina wrens, and probably some of the titmice
and woodpeckers are apparently almost or entirely nonmigratory. These
species may live out their entire existence without going more than
10 miles from the nest where they were hatched.
Many song sparrows, meadowlarks, blue jays, and other species make
such short migrations that the movement is difficult to detect
because individuals, possibly not the same ones, may be found in
one area throughout the year while other individuals that move
south may be replaced by individuals from the north. Information on
different movements of this type, within a species, can be gained by
observing birds marked with numbered bands, colored materials, or
identification of racially distinct museum specimens.
The American robin is a good example of this type of movement. This
species occurs in the southern United States throughout the year,
but in Canada and Alaska only during the summer. Its movements are
readily ascertained from study specimens. The breeding robin of the
southeastern states is the southern race. In autumn most of its more
northern nesters, such as those from Maryland and Virginia move into
the southern part of the breeding range or slightly farther south.
At about the same time the northern American robin moves south and
winters throughout the breeding and wintering range of its smaller
and paler southern relative. Thus there is complete overlap of
wintering ranges of northern and southern American robin populations,
although some individuals of the northern race winter in areas
vacated earlier by the southern race.
Among many migratory species there is considerable variation among
individuals and populations with respect to distances moved. Certain
populations may be quite sedentary while others are strongly
migratory, and certain individuals of the same population can be more
migratory than others. For example, red-winged blackbirds nesting
on the Gulf Coast are practically sedentary, but in winter they are
joined by other subspecies that nest as far north as the Mackenzie
Valley. In certain populations of the song sparrow and other species,
males remain all year on their northern breeding grounds while the
females and young migrate south.
Several species containing more than one distinguishable population
exhibit "leap-frog" migration patterns. The familiar eastern fox
sparrow breeds from northeastern Manitoba to Labrador, but during
the winter it is found concentrated in the southeastern part of the
United States. On the west coast of the continent, however, a study
of museum specimens by Swarth (1920), indicated six subspecies of
this bird breeding in rather sharply delimited ranges extending from
Puget Sound and Vancouver Island to Unimak Island, at the end of the
Alaskan Peninsula. One of these subspecies, known as the sooty fox
sparrow, breeds from the Puget Sound-Vancouver Island area northward
along part of the coast of British Columbia. It hardly migrates at
all, while the other races, nesting on the coast of Alaska, are
found in winter far to the south in Oregon and California. Although
much overlap exists, the races breeding farthest north generally
tend to winter farthest south. This illustrates a tendency for those
populations forced to migrate to pass over those subspecies so
favorably located as to be almost sedentary. If the northern birds
settled for the winter along with the sedentary population, winter
requirements may not be as sufficient as in the unoccupied areas
farther south (Fig. 10). Therefore, natural selection has insured
the different populations will survive the winter by separating the
subspecies into different wintering areas.
Another example of this "leap-frog" migration is illustrated by the
common yellowthroat of the Atlantic coast. Birds occupying the most
southern part of the general range are almost nonmigratory and reside
throughout the year in Florida, whereas the population that breeds
as far north as Newfoundland goes to the West Indies for the winter.
Thus the northern population literally "jumps" over the home of the
southern relatives during migratory journeys.
The palm warbler breeds from Nova Scotia and Maine west and northwest
to southern Mackenzie. The species has been separated into two
subspecies: those breeding in the interior of Canada and those
breeding in northeastern United States and Canada. The northwestern
subspecies makes a 3,000-mile journey from Great Slave Lake to Cuba
and passes through the Gulf States early in October. After the bulk
of these birds have passed, the eastern subspecies, whose migratory
journey is about half as long, drifts slowly into the Gulf Coast
region and remains for the winter.
Fall Flights Not Far South of Breeding Range
Some species have extensive summer ranges (e.g., the pine warbler,
rock wren, field sparrow, loggerhead shrike, and black-headed
grosbeak) and concentrate during the winter season in the southern
part of the breeding range or occupy additional territory only a
short distance farther south. The entire species may thus be confined
within a restricted area during winter, but with the return of warmer
weather, the species spreads out to reoccupy the much larger summer
range.
[Illustration: _Figure 10. Migration of Pacific coast forms of the
fox sparrow. The breeding ranges of the different races are encircled
by solid lines, while the winter ranges are dotted. The numbers
indicate the areas used by the different subspecies as follows: L
Shumagin fox sparrow; 2. Kodiak fox sparrow; 3. Valdez fox sparrow;
4. Yakutat fox sparrow; 5. Townsend fox sparrow; 6. sooty fox sparrow
(After Swarth 1920)._]
Many species, including the tree sparrow, snow bunting, and Lapland
longspur, nest in the far north and winter in the eastern United
States, while others, including the vesper and chipping sparrows,
common grackle, red-winged blackbird, eastern bluebird, American
woodcock, and several species of ducks, nest much farther south
in the United States and Canada and move south a relatively short
distance for the winter to areas along the Gulf of Mexico. In a
few of the more hardy species, individuals may linger in protected
places well within reach of severe cold. The common snipe, for
example, is frequently found during subzero weather in parts of the
Rocky Mountain region where warm springs assure a food supply. More
than 100 summer birds leave the United States entirely and spend
the winter in the West Indies, Central America, or South America.
For example, the Cape May warbler breeds from northern New England,
northern Michigan, and northern Minnesota, north to New Brunswick,
Nova Scotia, and nearly to Great Slave Lake. In winter it is
concentrated chiefly in the West Indies on the island of Hispaniola.
Long Distance Migration
Some of the common summer residents of North America are not content
with a trip to northern tropical areas of the West Indies and Central
America, but push on across the Equator and finally come to rest for
the winter in Patagonia or the pampas of Argentina. Species such as
nighthawks, some barn swallows, cliff swallows, and a few thrushes
may occupy the same general winter quarters in Brazil, but other
nighthawks and barn swallows go farther south. Of all North American
landbirds these species probably travel the farthest; they are found
north in summer to the Yukon Territory and Alaska, and south in
winter to Argentina, 7,000 miles away. Such seasonal flights are
exceeded in length, however, by the remarkable journeys of several
species of shorebirds including white-rumped and Baird's sandpipers,
greater yellowlegs, turnstones, red knots, and sanderlings. In this
group, 19 species breed north of the Arctic Circle and winter in
South America; six of these go as far south as Patagonia, a distance
of over 8,000 miles.
The Arctic tern is the champion "globe trotter" and long-distance
flier (Fig. 11). Its name "Arctic" is well earned, as its breeding
range is circumpolar and it nests as far north as the land extends in
North America. The first nest found in this region was only 7½° (518
miles) from the North Pole and contained a downy chick surrounded
by a wall of newly fallen snow scooped out by the parent. In North
America the Arctic tern breeds south in the interior to Great Slave
Lake, and on the Atlantic coast to Massachusetts. After the young are
grown, the Arctic terns disappear from their North American breeding
grounds and turn up a few months later in the Antarctic region,
11,000 miles away. For a long time the route followed by these hardy
fliers was a complete mystery; although a few scattered individuals
have been noted south as far as Long Island in the United States, the
species is otherwise practically unknown along the Atlantic coasts of
North America and northern South America. It is, however, known as a
migrant on the west coast of Europe and Africa. By means of numbered
bands, a picture disclosed what is apparently not only the longest,
but also one of the most remarkable migratory journeys (Austin 1928).
[Illustration: _Figure 11. Distribution and migration of arctic
terns. The route indicated for this bird is unique, because no other
species is known to breed abundantly in North America and to cross
the Atlantic Ocean to and from the Old World. The extreme summer and
winter homes are 11,000 miles apart._]
Few other animals in the world enjoy as many hours of daylight as the
Arctic tern. For these birds, the sun never sets during the nesting
season in the northern part of the range, and during their winter
sojourn to the south, daylight is continuous as well. In other months
of the year considerably more daylight than darkness is encountered.
=ORIENTATION AND NAVIGATION=
There probably is no single aspect of the entire subject of bird
migration that increases our admiration so much as the unerring
certainty with which birds cover thousands of miles of land and
water to come to rest in exactly the same spot where they spent the
previous summer or winter. Records from birds marked with numbered
bands offer abundant proof that the same individuals of many species
will return again and again to identical nesting or winter feeding
sites.
This ability to travel with precision over seemingly featureless
stretches of land or water is not limited to birds but is likewise
possessed by certain mammals, reptiles, fishes, and insects; the
well-known migrations of salmon and eels are notable examples.
For an animal to return to a specific spot after a lengthy migration,
it must use true navigation to get there. That is, it needs to not
only travel in a given compass heading and know where it is at any
given time so the course may be altered when necessary but also be
able to recognize its goal when it has arrived. It is dangerous to
generalize on the means of orientation and navigation in migration;
different groups of birds with different modes of existence have
evolved different means of finding their way from one place to
another (Pettingill 1970). We are only beginning to realize the
complexities involved in the many modes of bird orientation and
navigation. All we can do in this section is present a brief summary
of some of the more important principles involved and the studies
that have enhanced our knowledge in the area.
Ability to follow a more or less definite course to a definite goal
is evidently part of an inherited faculty. Both the direction and the
goal must have been implanted in the bird's genetic code when the
particular population became established at its present location.
The theory is sometimes advanced that older and more experienced
birds lead the way and thereby show the route to their younger
companions. This explanation may be acceptable for some species such
as geese, swans, and cranes because they stay in family groups, but
not for species in which adults and young are known to migrate at
different times, especially when young migrate ahead of the adults.
As indicated in a previous section on segregation, many North
American shorebirds as well as the cuckoos of New Zealand do this.
An inherited response to its surroundings, with a definite sense of
the goal to be reached and the direction to be followed, must be
attributed to these latter birds.
It is well known that birds possess wonderful vision. If they also
have retentive memories subsequent trips over the route may well
be steered in part by recognizable landmarks. Arguments against
the theory of landmark memory are chiefly that unescorted young
birds, without previous experience, can find their way to the winter
quarters of their species, even if the wintering area has a radically
different landscape and vegetation than the breeding grounds.
Experimental findings and field observations indicate landmarks are
used in navigation by certain birds, but the degree of use varies
considerably among the species (Bellrose 1972a).
To a land-dweller traveling the ocean, the vast expanse may seem
featureless but the reverse may be true for a seabird blown over land
by a storm. In the latter situation the differences in vegetation
and topography "obvious" to land-dwellers are completely foreign to
a seabird as it has had little previous experience to help interpret
these "strange objects." Griffin and Hock (1949) observed the flight
behavior of gannets displaced far inland away from their nests.
The bird appeared to search randomly until the coastline was met,
then the fliers pursued a much more direct course home. Herring
gulls, displaced about 250 miles from their nest in 2 consecutive
years, returned the second year in one-sixth the time required the
first year (Griffin 1943). To birds such as gannets, albatrosses,
and shearwaters, which spend almost their entire lives traveling
thousands of miles at sea and return to very specific nesting areas,
the "featureless ocean expanses" are probably very rich in visual
cues. It is difficult to believe a bird dependent on the sea for
its livelihood cannot help but be aware of wave direction, islands,
reefs, atolls, concentrations of floating flotsam, organisms,
currents, clouds over islands, fog belts, etc.
Much migration takes place at night and great stretches of the open
sea are crossed to reach destinations. Nights are rarely so dark that
all terrestrial objects are totally obscured, and features such as
coastlines and rivers are just those that are most likely to be seen
in the faintest light, particularly by the acute vision of birds from
their aerial points of observation. Even if terrestrial objects are
completely obscured on a very dark night, the migrants are still able
to assess their surroundings during the day before starting out again.
Some birds, especially colonial seabirds, seem to be able to fly
unerringly through the densest fog, particularly in the vicinity of
their nest site. Members of the Biological Survey, proceeding by
steamer through a dense fog from the island of Unalaska to Bogoslof
Island in the Bering Sea, recorded flocks of murres, returning to
Bogoslof after quests for food. The birds broke through the wall of
fog astern, flew by the vessel, and disappeared into the mists ahead
on the same course as the ship. On the other hand, radar observations
of migrating birds have indicated strong directional movements
on clear nights but often completely random movements in heavily
overcast or stormy weather. Possibly some birds can perceive the
position of the sun through an overcast as honey bees are known to
do. It is less likely the stars could be detected through overcast at
night.
Careful studies have been made on the homing instinct in various
seabirds such as Laysan albatrosses, Manx shearwaters, and several
tropical species of terns. Sooty and noddy terns reach their most
northern breeding point on the Dry Tortugas, off the southwest coast
of Florida. They are not known to wander any appreciable distance
farther north. Displaced breeding birds returned to their nests on
the Dry Tortugas after they had been taken on board ship, confined
in cages below decks, and carried northward 400 to 800 miles before
being released in a region where they had had no previous experience.
Likewise, Laysan albatrosses and Manx shearwaters have returned over
3,000 miles in similar homing experiments.
Possibly the "homing instinct," as shown by pigeons, terns,
shearwaters, albatrosses, and by the frigatebirds trained as message
carriers in the South Pacific, may not be identical with the sense
of perceptive orientation that figures in the flights of migratory
birds. Nevertheless, it seems closely akin and is probably governed
by the same mechanisms. There are good reasons to assume that once
we know the processes governing displaced homing we will know, in
general, how birds navigate; this question is still far from being
answered (Wallraff 1967).
Some students have leaned toward the possible existence of a
"magnetic sense" as being the important factor in the power of
geographical orientation. The theory was conceived as early as 1855
and reported in 1882 by Viguier. Investigations of this have been
conducted by Yeagley (1947) and Gordon (1948) with contradictory
results. In 1951, Yeagley incorporated the idea that sensitivity
to the effect of the earth's rotary motion through the vertical
component of the magnetic field is the means of orientation. The
basic question asked of the theory is: "Can birds detect such minute
differences in the earth's magnetic field and can these forces affect
bird behavior?"
Attempts to demonstrate the effect of radio waves on the navigational
ability of birds have produced contradictory results. In some of
these tests, homing pigeons released near broadcasting stations have
appeared to be hopelessly confused, whereas in others, apparently
conducted in the same manner, no effects could be discerned. Before
sensitivity of birds to electromagnetic stimuli of any kind can be
accepted or rejected, much additional experimental work is necessary.
Human navigators have used the heavenly bodies in determining their
course and position for centuries. It would not be surprising then
to find other long-distance travelers using the same method. One of
the most constant visual cues a migrating bird could use would be the
sun's or moon's path and the location of the stars.
Some of the more recent experimental work on bird navigation has
been with astronomical (sun) and celestial (star) directional clues.
Studies by Kramer, Sauer, and others have indicated a phenomenal
inherited ability in birds to use the position of the sun by day and
the stars by night to chart their courses. This involves an intricate
compensation for daily, seasonal, and geographical changes in the
positions of these heavenly bodies. Kramer (1957, 1961) placed
diurnal migrants in circular cages and "changed" the position of the
sun with mirrors. The birds shifted their position to compensate
for these changes. Sauer (1957, 1958), in a fascinating study with
nocturnal migrant warblers, placed birds in a round cage open to the
sky. These birds oriented in the normal direction for that locality
and time of year. He next placed the cage and birds in a planetarium
and projected overhead the night sky star patterns for different
seasons and localities. The familiar star pattern produced a normal
orientation but an unfamiliar sky caused confusion and complete
disorientation. These experiments, begun in Germany, are still
continuing in other countries with other species. Emlen (1969) used
photoperiod manipulation to change the physiological states of spring
and fall migratory readiness in indigo buntings. Half the sample of
birds were in breeding condition whereas the other half were already
past the reproductive stage even though it was spring "outside."
When these birds were subjected to a spring star pattern in a
planetarium, the birds in spring condition oriented northward but
those in autumnal condition oriented southward. Although some results
have been negative, by and large the evidence supports the original
findings that the sun and stars are visual "landmarks" used by at
least some birds as well as bees and probably many other creatures in
finding their way home as well as to their winter and summer quarters.
In conclusion, then, we can say this about bird orientation and
navigation: 1) many cues are available to birds for migratory
guidance and one or several of these may be used by any migrant; 2)
different species and groups of birds use different cues, depending
on their migration traits; 3) visual cues probably play a predominant
role in migration (radar studies have indicated that some birds can
maintain their orientation even under completely overcast nights,
although they usually become disoriented under such conditions); and
4) long-distance migrants and pelagic species have a much higher
developed sense of orientation than those species that migrate only
short distances or not at all.
=INFLUENCE OF WEATHER=
It is thought by some that the weather has little to do with the
time of arrival of migratory birds. It is assumed that if the bird
is physiologically prepared for migration it departs, irrespective
of the weather. Even if this were the case, weather can influence
the progress of migration by not only controlling the advance of the
seasons but also by helping, hindering, or even stopping bird flight
(Welty 1962).
Some scientists believe that birds not only avoid bad weather at the
start of a journey but usually finish the journey in good weather
(Nesbit and Drury 1967b). Contrary to what many observers believe,
the arrival of birds in an area, whether they stop or continue on,
is more often controlled by the weather at the point of departure
than at the point of arrival. During the peak of migration, suitable
weather may occur at an observation site, but strong migratory
movements may be arrested before the birds arrive there because the
weather was not suitable at the point of departure or somewhere
in between. In addition, if there is good weather at the point of
departure as well as farther down the migration route, the migrants,
once air-borne in a favorable weather pattern, may continue on right
over an expectant observer and the whole flight will be missed.
Nesbit and Drury's (1967b) radar study on air-ground comparisons
found, with few exceptions, ground observers missed the largest
movements observed on radar. Observation of a large wave of arrivals
indicated migrants had been stopped by a meteorological barrier,
and people were actually not reporting maximum migration but an
interruption to migration. Therefore, when migration is proceeding
normally under safe conditions, very little movement is visible to
the ground observer but a large arrival of birds on the ground often
indicates something is not in order and the migrants have been forced
to stop for one reason or another.
The question is frequently asked: "How can I identify weather
conditions suitable or unsuitable for migration?" It is almost
impossible to discuss separately the effects of different weather
factors on migration because barometric pressure, temperature, wind,
and other meteorological phenomena are very closely related.
On the North American continent, air masses generally proceed about
600 miles per day from the west to the east. These air masses vary
in pressure, temperature, humidity, and wind. The wind within
these masses travels in either a clockwise (anticyclonic) or
counter-clockwise (cyclonic) direction. Cyclonic air masses contain
relatively moist warm air with low barometric pressure centers and
are designated "lows"; anticyclonic air masses are characterized
by dry cool air with high barometric pressure areas and are called
"highs." Where these air masses meet, a "front" is formed, and the
rapidity with which this front moves through an area depends on the
temperature and pressure gradient on either side of the front.
An understanding of frontal systems, with their associated wind,
temperature and humidity, is one of the keys to understanding when
birds migrate. You must not only watch the fronts in your area but
the progress of nearby air masses as well because the birds migrating
through your area have started their journey to the north or south
of you depending on the season. The weather conditions at point of
departure will dictate if and when birds will be passing through your
area in the near future.
[Illustration: _Figure 12. A hypothetical weather system that could
be ideal for mass migrations of waterfowl in the fall. The strong
southerly flow of air created by counter-clockwise winds about the
lows and the clockwise rotation of air about the highs, aids the
rapid movement of waterfowl from their breeding grounds in the
Canadian prairies to wintering areas in southern United States._]
During fall migration, the best passage of migrants usually occurs
2 days after a cold front has gone through. That is, the low has
passed and it is being followed by a high characterized by dropping
temperatures, a rising barometer, and clearing skies. The 24 hours
just after a low has passed are not always conducive to a good
passage of birds because winds are often too strong and turbulent in
the trough between the two air masses. Hochbaum (1955) correlated
mass movements of ducks through the prairies with weather systems
and noted the combination of weather conditions described above was
ideal for mass migrations of ducks during November. During this
period, observers at Delta, Manitoba, south to Louisiana recorded a
tremendous flight of ducks as the proper conditions of barometric
pressure, temperature, wind, and cloud cover passed across the
central United States and Canada. An example of the type of weather
system that is often associated with mass movements is illustrated in
Fig. 12.
Records of lapwings on Newfoundland and the Gulf of St. Lawrence
appear to be the result of a particular series of meteorological
events (Bagg 1967). The lapwing is a European species rarely found
in the New World. If cold air moves into western Europe from the
east, lapwings move westward into England, Wales, and Ireland.
Occasionally, the development of an anomalous weather pattern
over the North Atlantic including an elongated low from Europe to
eastern Canada causes some birds to be literally "blown" in the
counter-clockwise airstream across the Atlantic to the Gulf of St.
Lawrence.
During spring migration, weather conditions conducive to strong
movements of birds are somewhat the opposite from those in the fall.
Migrants will move north on the warm sector of an incoming low.
When a high pressure area has just passed, the influx of warm moist
tropical air is extended and intensified (Bagg et al. 1950). However,
during this time, cloudiness and rain associated with the low may
curtail migration or squeeze it into a narrow period proceeding along
the warm front. If a fast moving cold front approaches from the
northwest, the rapid movement of migrants will be sharply curtailed
or even grounded until more favorable conditions occur.
The incessant crescendo note of the ovenbird is ordinarily associated
with the full verdure of May woods, but this bird has been known
to reach its breeding grounds in a snowstorm, and the records of
its arrival in southern Minnesota show a temperature variation
from near freezing to full summer warmth. Temperatures at arrival
of several other common birds vary from 14° between highest and
lowest temperatures to 37°, the average variation being about 24°.
North American species spending the winter months in tropical
latitudes experience no marked changes in temperature conditions
from November to March or April, yet frequently they will start the
northward movement in January or February. This is in obedience
to physiological promptings and has no relation to the prevailing
weather conditions. For migratory birds the winter season is a
period of rest, a time when they have no cares other than those
associated with the daily search for food or escape from their
natural enemies. Their migrations, however, are a vital part of their
life cycles, which have become so well adjusted that the seasons
of travel correspond in general with the major seasonal changes on
their breeding grounds. With the approach of spring, therefore,
the reproductive impulse awakens, and each individual bird is
irresistibly impelled to start the journey that ends in its summer
home.
In other words, the evidence indicates the urge to migrate is so
innate within a species or population that the individuals move
north in spring when the average weather is not unendurable. The
word "average" must be emphasized since it appears the migrations of
birds have evolved in synchrony with average climatic conditions.
More northern nesting populations of species such as American robins
and savannah sparrows, timed to arrive on their breeding ground when
the weather is suitable, pass through areas where their more southern
kin are already nesting. The hardy species travel early, fearless of
the blasts of retreating winter, while the more delicate kinds come
later when there is less danger of encountering prolonged periods of
inclement weather. Some of the hardy birds pause in favorable areas
and allow the spring season to advance. Then, by rapid travel they
again overtake it, or, as sometimes happens, they actually outstrip
it. Occasionally this results in some hardship, but rarely in the
destruction of large numbers of individuals after arrival. Cases are
known where early migrating bluebirds have been overwhelmed by late
winter storms. Nevertheless, if such unfavorable conditions are not
prolonged, no serious effect on the species is noted. The soundness
of the bird's instincts is evidenced by the fact that natural
catastrophes, great though they may be, do not permanently diminish
the avian populations.
The spring flight of migrants, if interrupted by cold north winds,
is resumed when weather conditions again become favorable, and it
is probable that all instances of arrival in stormy weather can be
explained on the theory that the flight was begun while the weather
was auspicious. Even though major movements of migrants in spring
generally coincide with periods of warm weather and southerly winds,
observations on the beginning of nocturnal spring flights from the
coast of Louisiana failed to note any inhibiting factor other than
hard rain (Gauthreaux 1971).
Radar studies have indicated that migrant birds possess an amazing
understanding of wind patterns (Bellrose 1967). Birds can recognize
many characteristics and select for favorable patterns. Head
winds are as unfavorable to migration as is rain or snow because
they greatly increase the labor of flight and cut down the speed
of cross-country travel. If such winds have a particularly high
velocity, they may force down the weaker travelers, and when this
happens over water, large numbers of birds are lost. Moderate tail
winds and cross or quartering breezes appear to offer the best
conditions for the flight of migrants. Richardson (1971) found
migrants traveling in different directions at different altitudes,
but each group of birds was aided by a following wind. Thus we might
expect natural selection to operate in favor of those birds that
could recognize and respond to favorable wind patterns because it
would reduce energy consumption and flight time on long-distance
flights (Hassler et al. 1963).
Soaring birds such as hawks, vultures, and storks are very dependent
on proper wind conditions for migration. In the fall, often the best
day to observe hawk migration in the eastern United States is on the
second day after a cold front has passed providing there are steady
northwest to west winds and a sunny day for production of thermals
(Pettingill 1962). Considerable drifting may be observed in this
group of birds because they are literally carried along by the wind
or glide from one thermal to the next. Haugh and Cade (1966) found
most hawks migrated around Lake Ontario when winds were 10 to 25
miles per hour, but, if the wind exceeded 35 miles per hour, most
hawk migration stopped.
In conclusion then, we can say that the weather may be the impetus
for migration for many species, but it cannot stimulate a bird
to migrate unless it is physiologically prepared. Arrivals on
the ground are not necessarily indicative of the number of birds
passing overhead. During the fall, peak migrations usually follow
the passage of a cold front when the temperature is falling, the
barometer is rising, winds are from the west or northwest, and the
sky is clearing. In the spring, most migrants proceed north in the
warm sector of a low when winds are southerly, warm, and moist,
but rain, fog, or snow will often curtail the passage of migrants
or prevent the initiation of a migration. Evolution of migratory
behavior has probably resulted from the survival of birds capable of
selecting those wind conditions, which reduce flight time and energy
consumption, during their passage.
=INFLUENCE OF TOPOGRAPHY=
The relation of the world's land masses to each other and the
distribution and association of biotypes within these land masses
influence the direction birds migrate. Topography may aid, hinder, or
prevent the progress of a migrant depending on the bird's particular
requirements. Old World migrants must contend with east-west tending
mountain ranges and deserts, whereas New World travelers can proceed
north and south across a landscape with its major mountain ranges and
river systems oriented in the same direction as the birds migrate.
When a distinct feature in the landscape, such as borders between
fields and forests, rivers, mountain ridges, desert rims, or
peninsulas, appears to influence migratory travel, we call these
formations "guiding lines," "diversion-lines," "leading lines," or
in German, "Leitlinie." It is an observed fact that some birds in
a migratory movement alter their course to travel along a leading
line, but whether this feature in the landscape caused the migrants
to change their course is only theory (Thomson 1960). Besides
topography, many other factors can influence this type of flight
behavior including weather, wind speed and direction, time of day,
species, age, and experience of the bird (Murray 1964).
Large bodies of water constitute real barriers to soaring birds
dependent on thermals and air currents. Good examples of these
barriers include the Mediterranean Sea between Europe and Africa and
the Great Lakes in North America. Because these water areas do not
create good thermals (generally a warm surface, such as a large field
on a sunny day, is needed to create the necessary rising air currents
for thermals to form) for birds to soar on, migrants are forced to
travel around them on up-drafts created where land and water meet.
The shoreline, then, may appear to be the guiding line, but more
than likely the birds are simply following air currents created by
onshore winds replacing the rising air from the surrounding warmer
land surface and being deflected upward by the shoreline. These
conditions often concentrate our buteos (broad-winged, rough-legged,
red-shouldered, and red-tailed hawks) into restricted areas where, on
good days, numbers observed can be spectacular. Similar conditions
exist over the Bosphorus at the eastern end of the Mediterranean Sea
where literally thousands of storks, eagles, and buzzards can be
observed on a good day.
While extensive water areas may alter the migratory path of soaring
birds, mountain ridges, especially if parallel to the line of flight,
are often very conducive to migratory travel. Systematic coverage
of the Appalachian ridges indicates all of them aid the migration
of soaring birds. Apparently the highest and longest ridges deflect
the horizontal winds upward better than the shorter ridges less than
1,000 feet high, and more birds are seen, on the average, along the
higher ridges (Robbins 1956).
In general, nocturnal migrants are not influenced by topography
as much as diurnal travellers. Radar observations have played an
important role in establishing this difference. Bellrose (1967)
found that waterfowl migrating at night through the Midwest were
not influenced by major river systems, but in the evening or
after daybreak ducks and geese tended to alter their course along
the rivers. Drury et al. (1961) recorded massive fall and spring
movements from the New England area out over the Atlantic Ocean
without any apparent regard for the coastline. Until nocturnal
migration could be "watched" on a radar screen, many bird observers
assumed the guiding effect of the coastline on migratory travel was
more restrictive than it really is.
In summary, topography may help or deter a migrant in its passage.
It affects different birds in different ways. In North America,
migratory movements are continent wide, and no evidence has indicated
any particular part of the landscape influences all birds in the same
manner. Certain bird populations may use general areas in migration,
but they are usually not rigidly restricted to them because of
topography.
=PERILS OF MIGRATION=
The migration season is full of peril for birds. Untold thousands of
smaller migrants are destroyed each year by storms and attacks by
predatory animals. These mortality factors, and others, help keep
bird populations in check. Perils of migration are among these causes.
Storms
Of all the hazards confronting birds in migration, particularly the
smaller species, storms are the most dangerous. Birds that cross
broad stretches of water can be blown off course by a storm, become
exhausted, and fall into the waves. Such a catastrophe was once seen
from the deck of a vessel in the Gulf of Mexico, 30 miles off the
mouth of the Mississippi River. Great numbers of migrating birds,
chiefly warblers, were nearing land after having accomplished nearly
95 percent of their long flight when, caught by a "norther" against
which they were unable to make headway, hundreds were forced into
the waters of the Gulf and drowned. A sudden drop in temperature
accompanied by a snowfall can cause a similar affect.
Aerial Obstructions
Lighthouses, tall buildings, monuments, television towers, and
other aerial obstructions have been responsible for destruction of
migratory birds. Bright beams of lights on buildings and airport
ceilometers have a powerful attraction for nocturnal air travelers
that may be likened to the fascination for lights exhibited by many
insects, particularly night-flying moths. The attraction is most
noticeable on foggy nights when the rays have a dazzling effect that
not only lures the birds but confuses them and causes their death by
collision against high structures. The fixed, white, stationary light
located 180 feet above sea level at Ponce de Leon Inlet (formerly
Mosquito Inlet), Florida, has caused great destruction of bird
life even though the lens is shielded by wire netting. Two other
lighthouses at the southern end of Florida, Sombrero Key and Fowey
Rocks, have been the cause of a great number of bird tragedies, while
heavy mortality has been noted also at some of the lights on the
Great Lakes and on the coast of Quebec. Fixed white lights seem to be
most attractive to birds; lighthouses equipped with flashing or red
lights do not have the same attraction.
For many years in Washington, B.C., the illuminated Washington
Monument, towering more than 555 feet into the air, caused
destruction of large numbers of small birds. Batteries of brilliant
floodlights grouped on all four sides about the base illuminate
the Monument so brilliantly, airplane pilots noticed that it could
be seen for 40 miles on a clear night. It is certain there is an
extensive area of illumination, and on dark nights with gusty,
northerly winds, nocturnal migrants seem to fly at lower altitudes
and are attracted to the Monument. As they mill about the shaft,
they are dashed against it by eddies of wind, and hundreds have been
killed in a single night.
In September 1948, bird students were startled by news of the
wholesale destruction of common yellowthroats, American redstarts,
ovenbirds, and others against the 1,250-foot-high Empire State
Building in New York City, the 491-foot-high Philadelphia Saving Fund
Society Building in Philadelphia, and the 450-foot-high WBAL radio
tower in Baltimore. In New York, the birds continued to crash into
the Empire State Building for 6 hours.
More recently, the television tower has become the chief hazard.
These structures are so tall, sometimes over 1,000 feet, they present
more of a menace than buildings or lighthouses. Their blinking lights
cause passing migrants to blunder into guy wires or the tower itself
while milling around like moths about a flame. Numerous instances
(e.g. Stoddard and Norris 1967) throughout the U.S. indicate this
peril to migration is widespread. The lethal qualities of airport
ceilometers have been effectively modified by conversion to
intermittent or rotating beams.
Exhaustion
Both soaring and sailing birds are so proficient in aerial
transportation that only recently have the principles been understood
and imitated by aircraft pilots. The use of ascending air currents,
employed by all soaring birds and easily demonstrated by observing
gulls glide hour after hour along the windward side of a ship, are
now utilized by man in his operation of gliders. Moreover, the whole
structure of a bird makes it the most perfect machine for extensive
flight the world has ever known. Hollow, air-filled bones, together
with feathers, the lightest and toughest material known for flight,
have evolved in combination to produce a perfect flying machine.
Mere consideration of a bird's economy of fuel or energy also is
enlightening. The golden plover probably travels over a 2,400-mile
oceanic route from Nova Scotia to South America in about 48 hours
of continuous flight. This is accomplished with the consumption of
less than 2 ounces of body fat (fuel). In contrast, to be just as
efficient in operation, a 1,000-pound airplane would consume only
a single pint of fuel in a 20-mile flight rather than the gallon
usually required. Similarly, the tiny ruby-throated hummingbird
weighing approximately 4 grams, crosses the Gulf of Mexico in a
single flight of more than 500 miles while consuming less than 1 gram
of fat.
One might expect the exertion incident to long migratory flights
would result in arrival of migrants at their destination near a state
of exhaustion. This is usually not the case. Birds that have recently
arrived from a protracted flight over land or sea sometimes show
evidences of being tired, but their condition is far from being in a
state of emaciation or exhaustion. The popular notion birds find long
ocean flights so excessively wearisome that they sink exhausted when
terra firma is reached generally does not coincide with the facts.
The truth is, even small landbirds are so little exhausted by ocean
voyages, they not only cross the Gulf of Mexico at its widest point
but may even proceed without pause many miles inland before stopping.
The sora, considered such a weak flyer that at least one writer was
led to infer most of its migration was made on foot, has one of the
longest migration routes of any member of the rail family and even
crosses the wide reaches of the Caribbean Sea. Observations indicate
that under favorable conditions birds can fly when and where they
please and the distance covered in a single flight is governed
chiefly by the amount of stored fat. Exhaustion, except as the result
of unusual factors such as strong adverse winds, cannot be said to be
an important peril of migration.
=ROUTES OF MIGRATION=
General Considerations
While it is beyond question that certain general directions of
flight are consistently followed by migratory birds, it is well to
remember the term "migration route" is to some extent a theoretical
concept referring to the lines of general advance or retreat of a
species, rather than the exact course followed by individual birds or
a path followed by a species with specific geographic or ecological
boundaries. Even the records of banded birds usually show no more
than the place of banding and recovery. One ought to have recourse to
intermediate records and reasoning based on probabilities to fill in
details of the route actually traversed between the two points. In
determining migration routes, one must constantly guard against the
false assumption that localities with many grounded migrants are on
the main path of migration and localities where no grounded migrants
are observed are off the main path.
There is also infinite variety in the routes covered during migration
by different species. In fact, the choice of migration highways is
so wide that is seems as if the routes of no two species coincide.
Differences in distance traveled, time of starting, speed of flight,
geographical position, latitudes of breeding and wintering grounds,
and other factors contribute to this great variation of migration
routes. Nevertheless, there are certain factors that serve to guide
individuals or groups of individuals along more or less definite
lines, and it is possible to define such lines of migration for many
species.
Except in a few species, individuals probably do not follow precisely
the same route twice. This is especially true in the group of
soaring birds that utilize thermals. Mueller and Berger (1967b)
recaptured only three migrants in subsequent years at Cedar Grove,
Wisconsin, after banding over 50,000 birds there. In general, those
populations of species with very discernible breeding or wintering
grounds have readily discernible migration routes. However, even the
whole migration process of certain species may show great yearly
fluctuations (Rudebeck 1950).
Aldrich et al. (1949) showed from banding data great variation in
migration patterns between species of waterfowl. In some species
there was considerable diversity in direction of movement, not only
of different breeding populations within a species but also for
different individuals of the same breeding population. The impression
is inescapable; waterfowl migration is even more complicated than
originally supposed, and it is difficult to make generalizations
with regard to migration pathways for even a single species let
alone waterfowl in general.
Flyways and Corridors
Through plotting accumulated banding data in the 1930's,
investigators became impressed by what appeared to be four broad,
relatively exclusive flyway belts in North America. This concept,
based upon analyses of the several thousand records of migratory
waterfowls recoveries then available, was described by Lincoln
(1935a). In this paper (p. 10), Lincoln concluded that:
... because of the great attachment of migratory birds for their
ancestral flyways, it would be possible practically to exterminate
the ducks of the West without seriously interfering with the supply
of birds of the same species in the Atlantic and Mississippi
flyways, and that the birds of these species using the eastern
flyways would be slow to overflow and repopulate the devastated
areas of the West, even though environmental conditions might be so
altered as to be entirely favorable.
Since 1948, this concept served as the basis for administrative
action by the Fish and Wildlife Service in setting annual migratory
waterfowl hunting regulations.
The concept of bird populations being confined to four fairly
definite and distinct migration "flyways" is probably most applicable
to those birds that migrate in family groups, namely geese, swans,
and cranes, but does not appear to be very helpful in understanding
the movements of the more widely dispersing ducks. The "pioneering
spirit" in Canada geese, for example, is limited by their social
structure the young travel to and from specific breeding and
wintering areas with their parents. These young later in life usually
breed in the same areas as did their parents. If a goose population
is decimated in one flyway, either by hunting or natural calamities,
other goose populations in other flyways are not seriously
endangered, but also these populations are very slow to repopulate
an area where the previous goose population had been decimated. This
is not the case with ducks because these birds are not always bound
by their intrinsic behavior to return to specific breeding areas.
Consequently, vacant breeding areas are more rapidly repopulated by
ducks than by geese.
Although Lincoln's analysis was confined to ducks and geese, some
thought that it applied to other groups of birds as well. Everyone
now realizes that the concept of four flyways, designated as
the Atlantic, Mississippi, Central, and Pacific Flyways, was an
oversimplification of an extremely complex situation involving
crisscrossing of migration routes, varying from species to species.
It can be considered meaningful only in a very general way, even
for waterfowl, and not applicable generally to other groups of
birds. Nevertheless the four "Flyway" areas have been useful in
regionalizing the harvest of waterfowl for areas of different
vulnerability of hunting pressure.
Bellrose (1968) identified corridors of southward migrating waterfowl
east of the Rocky Mountains and determined, through statistical
analyses, the relative abundance of birds in each. He showed major
corridors of dabbling duck movements down the Great Plains and
Missouri-Mississippi river valleys with minor off shoots at various
points from these corridors eastward to the Atlantic coast where they
joined equally minor eastern movements from the North (Fig. 13).
Bellrose's map of migration corridors for the diving ducks showed
heavy traffic similar to that of dabbling species down the Great
Plains and relatively heavily used corridors from these central
arteries eastward across the Great Lakes area to the Atlantic coast,
terminating particularly in the vicinity of Chesapeake Bay. A fairly
well-used corridor extends along the Atlantic coast.
[Illustration: _Figure 13. Migration corridors used by dabbling
ducks east of the Rocky Mountains during their fall migration (After
Bellrose 1968)._]
[Illustration: _Figure 14. Distribution and migration of Harris'
sparrow. This is an example of a narrow migration route through the
interior of the country._]
With our present knowledge of bird migration it is difficult at
best to recognize distinct broad belts of migration down the North
American continent encompassing groups of distinct populations or
species. It seems that so much intermingling of populations occurs
that distinctions between broad "flyway" belts are not discernible.
About all we can say for sure now is that birds travel between
certain breeding areas in the North and certain wintering areas in
the South and that a few heavily traveled corridors used by certain
species, and more generalized routes followed by one or more species,
have become obvious.
Narrow Routes
Some species exhibit extremely narrow routes of travel. The red knot
and purple sandpiper, for example, are normally found only along the
coasts because they are limited on one side by the broad waters of
the ocean, and on the other by land and fresh water; neither of these
habitats furnish conditions attractive to these species.
The Ipswich race of the savannah sparrow likewise has a very
restricted migration range. It is known to breed only on tiny Sable
Island, Nova Scotia, and it winters from that island south along the
Atlantic coast to Georgia. It is rarely more than a quarter of a mile
from the outer beach and is entirely at home among the sand dunes
with their sparse covering of coarse grass.
The Harris' sparrow supplies an interesting example of a moderately
narrow migration route in the interior of the country (Fig. 14).
This fine, large sparrow is known to breed only in the narrow belt
of stunted timber and brush at or near the limit of trees from the
vicinity of Churchill, Manitoba, on the west shore of Hudson Bay, to
the Mackenzie Delta 1,600 miles to the northwest. When this sparrow
reaches the United States on its southward migration, it is most
numerous in a belt about 500 miles wide, between Montana and central
Minnesota and continues south through a relatively narrow path in the
central part of the continent. Knowledge of habitat preference by
Harris' sparrows suggests the narrow migration range is restricted
to the transition between woodland and prairie, a type of habitat
approaching the woodland-tundra transition of its breeding area.
Development of this migration route, of course, preceded destruction
of the heavy eastern forests by colonists from Europe. Its winter
range lies primarily in similar country extending from southeastern
Nebraska and northwestern Missouri, across eastern Kansas and
Oklahoma and through a narrow section of eastern Texas, at places
hardly more than 150 miles wide.
Converging Routes
When birds start their southward migration the movement necessarily
involves the full width of the breeding range. Later, in the case
of landbirds with extensive breeding ranges, there is a convergence
of the lines of flight taken by individual birds owing, in part,
to the conformation of the land mass and in part to the east-west
restriction of habitats suitable to certain species. An example of
this is provided by the eastern kingbird, which breeds in a summer
range 2,800 miles wide from Newfoundland to British Columbia. On
migration, however, the area traversed by the species becomes
constricted until in the southern part of the United States the
occupied area extends from Florida to the mouth of the Rio Grande, a
distance of only 900 miles. Still farther south the migration path
continues to converge, and, at the latitude of Yucatan, it is not
more than 400 miles wide. The great bulk of the species probably
moves in a belt less than half this width.
[Illustration: _Figure 15. Distribution and migration of the scarlet
tanager. During the breeding season individual scarlet tanagers may
be 1,500 miles apart in an east-and-west line across the breeding
range. In migration, however, the lines gradually converge until in
South America they are about 500 miles apart._]
The scarlet tanager presents another extreme case of a narrowly
converging migration route starting from its 1,900-mile-wide breeding
range in the eastern deciduous forest between New Brunswick and
Saskatchewan (Fig. 15). As the birds move southward in the fall,
their path of migration becomes more and more constricted, until,
at the time they leave the United States, all are included in the
600-mile belt from eastern Texas to the Florida peninsula. The
boundaries continue to converge through Honduras and Costa Rica where
they are not more than 100 miles apart. The species winters in the
heavily forested areas of northwestern South America including parts
of Colombia, Ecuador, and Peru.
[Illustration: _Figure 16. Distribution and migration of the
rose-breasted grosbeak. Though the width of the breeding range is
about 2,500 miles, the migratory lines converge until the boundaries
are only about 1,000 miles apart when the birds leave the United
States._]
The rose-breasted grosbeak also leaves the United States through
the 600-mile stretch from eastern Texas to Apalachicola Bay, but
thereafter as this grosbeak crosses the Gulf of Mexico and enters the
northern part of its winter quarters in southern Mexico the lines
do not further converge. However, the pathway of those individuals
that continue on to South America is considerably constricted by the
narrowing of the land through Central America to Panama (Fig. 16).
Although the cases cited represent extremes of convergence, a
narrowing of the migratory path is the rule to a greater or lesser
degree for the majority of North American birds. Both the shape of
the continent and major habitat belts tend to constrict southward
movement so that the width of the migration route in the latitude
of the Gulf of Mexico is usually much less than in the breeding
territory.
The American redstart represents a case of a wide migration route,
but even in the southern United States, this is still much narrower
than the breeding range (Fig. 17). These birds, however, cross all of
the Gulf of Mexico and pass from Florida to Cuba and Haiti by way of
the Bahamas, so here their route is about 2,500 miles wide.
[Illustration: _Figure 17. Distribution and migration of the
redstart. An example of a wide migration route, birds of this species
cross all parts of the Gulf of Mexico, or may travel from Florida to
Cuba and through the Bahamas. Their route has an east-and-west width
of more than 2,000 miles._]
Principal Routes From North America
W. W. Cook presented seven of the more important generalized routes
for birds leaving the United States on their way to various wintering
grounds (1915a; Fig. 18). When migrants return northward in the
spring, they may follow these same routes, but it is not known for
certain whether they do. These routes are discussed in the following
sections.
-------------------------------------------------
Atlantic Oceanic Route
-------------------------------------------------
Route No. 1 (Fig. 18) is almost entirely oceanic and passes directly
over the Atlantic Ocean from Labrador and Nova Scotia to the Lesser
Antilles, then through this group of small islands to the mainland
of South America. Most of the adult eastern golden plovers and some
other shorebirds use this as their fall route. As we mentioned
previously, radar has indicated strong fall movements of warblers
from the New England coast out over the Atlantic to points south
along this route. Since it lies almost entirely over the sea, this
route is definitely known only at its terminals and from occasional
observations made on Bermuda and other islands in its course. Some
of the shorebirds that breed on the Arctic tundra of the District of
Mackenzie (Northwest Territories) and Alaska fly southeastward across
Canada to the Atlantic coast and finally follow this oceanic route
to the mainland of South America. The golden plover may accomplish
the whole 2,400 miles without pause or rest, and in fair weather the
flocks pass Bermuda and sometimes even the islands of the Antilles
without stopping. Although most birds make their migratory flights
either by day or by night, the golden plover in this remarkable
journey flies both day and night. Since this plover swims lightly and
easily, it may make a few short stops along the way.
[Illustration: _Figure 18. Principal migration routes used by birds
in passing from North America to winter quarters in the West Indies,
Central America, and South America. Route 4 is the one used most
extensively while only a few species make the 2,400 mile flight down
Route 1 from Nova Scotia to South America._]
The Arctic tern follows the Atlantic Ocean route chiefly along the
eastern side of the ocean. Likewise, vast numbers of seabirds such
as auks, murres, guillemots, phalaropes, jaegers, petrels, and
shearwaters follow this over-water route from breeding coasts and
islands in the Northern and Southern Hemispheres.
-------------------------------------------------
Atlantic Coast Route and Tributaries
-------------------------------------------------
The Atlantic coast is a regular avenue of travel, and along it are
many famous points for observing both land and water birds. About 50
different kinds of landbirds that breed in New England follow the
coast southward to Florida and travel thence by island and mainland
to South America (Fig. 18, route 2). The map indicates a natural and
convenient highway through the Bahamas, Cuba, Hispaniola, Puerto
Rico, and the Lesser Antilles to the South American coast. Resting
places are afforded at convenient intervals, and at no time need the
aerial travelers be out of sight of land. It is not, however, the
favored highway; only about 25 species of birds go beyond Cuba to
Puerto Rico along this route to their winter quarters, while only
six species are known to reach South America by way of the Lesser
Antilles. Many thousands of American coots and wigeons, pintails,
blue-winged teal, and other waterfowl as well as shorebirds,
regularly spend the winter season in the coastal marshes, inland
lakes, and ponds of Cuba, Hispaniola, and Puerto Rico.
Route No. 3 (Fig. 18) is a direct line of travel for Atlantic coast
migrants en route to South America, although it involves much longer
flights. It is used almost entirely by landbirds. After taking off
from the coast of Florida there are only two intermediate land masses
where the migrants may pause for rest and food. Nevertheless, tens
of thousands of birds of about 60 species cross the 150 miles from
Florida to Cuba where many elect to remain for the winter months.
The others negotiate the 90 miles between Cuba and Jamaica, but,
from that point to the South American coast, there is a stretch of
islandless ocean 500 miles across. Relatively few North American
migrants on this route go beyond Jamaica. The bobolink so far
outnumbers all other birds using this route that it may be designated
the "bobolink route" (Fig. 19). As traveling companions along this
route, the bobolink may meet vireos, kingbirds, and nighthawks
from Florida, Chuck-will's-widows from the Southeastern States,
black-billed and yellow-billed cuckoos from New England, gray-cheeked
thrushes from Quebec, bank swallows from Labrador, and blackpoll
warblers from Alaska. Sometimes this scattered assemblage will be
joined by a tanager or a wood thrush, but the "bobolink route" is not
popular with the greater number of migrants.
[Illustration: _Figure 19. Distribution and migration of the
bobolink. In crossing to South America, most of the bobolinks use
route 3 (Fig. 18), showing no hesitation in making the flight from
Jamaica across an islandless stretch of ocean. It will be noted
that colonies of these birds have established themselves in western
areas, but in migration they adhere to the ancestral flyways and show
no tendency to take the short cut across Arizona, New Mexico, and
Texas._]
Formerly, it was thought most North America landbirds migrated
to Central America via the Florida coast, then crossed to Cuba,
and finally made the short flight from the western tip of Cuba to
Yucatan. A glance at the map would suggest this as a most natural
route, but, as a matter of fact, it is practically deserted except
for a few swallows and shorebirds or an occasional landbird
storm-driven from its normal course. What actually happens in the
fall is that many of the birds breeding east of the Appalachian
Mountains travel parallel to the seacoast in a more or less
southwesterly direction and, apparently maintaining this same general
course from northwestern Florida, cross the Gulf of Mexico to the
coastal regions of eastern Mexico. They thus join migrants from
farther inland in using route No. 4 (Fig. 18).
Routes used by the Atlantic brant merit some detail because their
flight paths were long misunderstood. These birds winter on the
Atlantic coast, chiefly at Barnegat Bay, New Jersey, but depending
upon the severity of the season and the food available, south also
to North Carolina. Their breeding grounds are in the Canadian arctic
archipelago and on the coasts of Greenland. According to the careful
studies of Lewis (1937), the main body travels northward in spring
along the coast to the Bay of Fundy, overland to Northumberland
Strait, which separates Prince Edward Island from mainland New
Brunswick and Nova Scotia. A minor route appears to lead northward
from Long Island Sound by way of the Housatonic and Connecticut River
Valleys to the St. Lawrence River.
After spending the entire month of May feeding and resting in the
Gulf of St. Lawrence, the eastern segment of the brant population
resumes its journey by departing overland from the Bay of Seven
Island area. The eastern and larger segment of the population appears
to fly almost due north to Ungava Bay and from there to nesting
grounds, probably on Baffin Island and Greenland. The smaller segment
travels a route slightly north of west to the southeastern shores
of James Bay, although east of that area some of the flocks take a
more northwesterly course by descending the Fort George River to
reach the eastern shore of James Bay. Upon their arrival at either
of these two points on James Bay, the brants of this western segment
turn northward and proceed along eastern Hudson Bay to their breeding
grounds in the Canadian Arctic.
In general, the fall migration of the brant follows the routes
utilized in the spring. At this season, the eastern population
appears only on the western and southern shores of Ungava Bay before
continuing their southward journey to the Gulf of St. Lawrence and
beyond. Also, it appears that most of the birds of the western
segment, instead of following the eastern shores of Hudson and James
bays, turn southwestward across the former, by way of the Belcher
Islands, to Cape Henrietta Maria, and from there south along the
western shores of James Bay by way of Akimiski and Charlton Islands.
At the southern end of James Bay, they are joined by those that have
taken the more direct route along the east coasts of the bays and all
then fly overland 570 miles to the estuary of the St. Lawrence River.
The Atlantic coast wintering area receives accretions of waterfowl
from three or four interior migration paths, one of which is of
first importance, as it includes great flocks of canvasbacks,
redheads, scaup, Canada geese, and many black ducks that winter in
the waters and marshes of the coastal region south of Delaware Bay.
The canvasbacks, redheads, and scaup coming from breeding grounds
on the great northern plains of central Canada follow the general
southeasterly trend of the Great Lakes, cross Pennsylvania over
the mountains, and reach the Atlantic coast in the vicinity of
Delaware and Chesapeake Bays. Black ducks, mallards, and blue-winged
teals that have gathered in southern Ontario during the fall leave
these feeding grounds and proceed southwest over a course that is
apparently headed for the Mississippi Valley. Many do continue this
route down the Ohio Valley, but others, upon reaching the vicinity
of the St. Clair Flats between Michigan and Ontario, swing abruptly
to the southeast and cross the mountains to reach the Atlantic coast
south of New Jersey. This route, with its Mississippi Valley branch,
has been fully documented by the recovery records of ducks banded at
Lake Scugog, Ontario.
Canvasbacks migrate from the prairie pothole country of the
central United States and Canada to many wintering areas in the
United States. This duck has been the subject of a particular
study (Stewart, Geis, and Evans 1958), and its principle migration
routes, based on recovery of banded birds, are shown to follow an
important trunk route from the major breeding area in the prairie
provinces of Canada and the northern prairies of the United States
southeastward through the southern Great Lakes area to Chesapeake
Bay, the chief wintering area (Fig. 20). Relatively few canvasbacks
proceed southward along the Atlantic seaboard. A less important route
branches off from the main trunk in the southern Minnesota region and
extends south along the Mississippi Valley to points along the river.
Other individuals of the prairie breeding population fly southward on
a broad front to the gulf coast of Texas and the interior of Mexico,
while some proceed southwestward on a relatively broad path to the
northern Pacific coast.
-------------------------------------------------
Mackenzie Valley-Great Lakes-Mississippi Valley
Route and Tributaries
-------------------------------------------------
The route extending from the Mackenzie Valley past the Great Lakes
and down the Mississippi Valley is easily the longest of any in the
Western Hemisphere. Its northern terminus is on the Arctic coast in
the regions of Kotzebue Sound, Alaska, and the mouth of the Mackenzie
River, while its southern end lies in Argentina. Nighthawks, barn
swallows, blackpoll warblers, and individuals of several other
species that breed northward to the Yukon Territory and Alaska must
cover the larger part of the route twice each year.
[Illustration: _Figure 20. Principal migratory routes of the
canvasback. The major route of travel extends from breeding areas in
central Canada southeast across the Great Lakes and either south down
the Mississippi River or east to Chesapeake Bay (After Stewart et al
1958)._]
For more than 3,000 miles--from the mouth of the Mackenzie to the
delta of the Mississippi--this route is uninterrupted by mountains.
In fact, the greatest elevation above sea level is less than 2,000
feet. Because it is well timbered and watered, the entire region
affords ideal conditions for its great hosts of migrating birds.
It is followed by such vast numbers of ducks, geese, shorebirds,
blackbirds, sparrows, warblers, and thrushes that observers stationed
at favorable points in the Mississippi Valley during the height of
migration can see a greater number of migrants than can be noted
anywhere else in the world.
When many of these species, including ducks, geese, robins, and
yellow-rumped warblers, arrive at the Gulf coast, they spread out
east and west for their winter sojourn. Others, despite the perils of
a trip involving a flight of several hundred miles across the Gulf
of Mexico, fly straight for Central and South America. This part of
the route is a broad "boulevard" extending from northwestern Florida
to eastern Texas and southward across the Gulf of Mexico to Yucatan
and the Isthmus of Tehuantepec (Fig. 18, route 4). This route appears
to have preference over the safer but more circuitous land or island
routes by way of Texas or Florida. During the height of migration
some of the islands off the coast of Louisiana are wonderful
observation points for the student of birds, as the feathered
travelers literally swarm over them.
Present detailed knowledge of the chief tributaries to the
Mackenzie-Great Lakes-Mississippi Valley route relates primarily to
waterfowl. Reference has been made already to the flight of black
ducks that reach the Mississippi Valley from southern Ontario. Some
individuals of this species banded at Lake Scugog, Ontario, have
been recaptured in succeeding seasons in Wisconsin and Manitoba, but
the majority was retaken at points south of the junction of the Ohio
River with the Mississippi indicating their main route of travel from
southern Ontario.
A second route that joins the main artery on its eastern side is
the one used by eastern populations of lesser snow geese, including
both blue and white phases, that breed mainly on Southampton Island
and in the Fox Basin of Baffin Island. In the fall these geese work
southward along the shores of Hudson Bay and, upon reaching the
southern extremity of James Bay, take off on their flight to the
great coastal marshes of Louisiana and Texas west of the Mississippi
River delta.
-------------------------------------------------
Great Plains-Rocky Mountain Routes
-------------------------------------------------
This route also has its origin in the Mackenzie River delta and
Alaska. The lesser sandhill cranes, white-fronted geese, and smaller
races of the Canada goose follow this route through the Great Plains
from breeding areas in Alaska and western Canada. It is used chiefly
by the pintails and American wigeons that fly southward through
eastern Alberta to western Montana. Some localities in this area, as
for example, the National Bison Range at Moiese, Montana, normally
furnish food in such abundance that these birds are induced to pause
in their migratory movement. Some flocks of pintails and wigeons move
from this area almost directly west across Idaho to the valley of the
Columbia River, then south to the interior valleys of California.
Others leave Montana by traveling southeastward across Wyoming and
Colorado to join other flocks moving southward through the Great
Plains.
Observations made in the vicinity of Corpus Christi, Texas, have
shown one of the short cuts (Fig. 18, route 5) that is part of the
great artery of migration. Thousands of birds pass along the coast
to the northern part of the State of Veracruz, Mexico. Coastal areas
along the State of Tamaulipas to the north are arid and so entirely
unsuited for frequenters of moist woodlands that it is probable
that much, or all, of this part of the route for these species is
a short distance off shore. It is used by such woodland species as
the golden-winged warbler, the worm-eating warbler, and the Kentucky
warbler.
-------------------------------------------------
Pacific Coast Route
-------------------------------------------------
Although it does present features of unusual interest, the Pacific
coast route is not as important as some of the others described.
Because of the equable conditions that prevail, many species of birds
along the coast from the northwestern states to southeastern Alaska
either do not migrate or else make relatively short journeys. This
route has its origin chiefly in western Alaska, around the Yukon
River delta. Some of the scoters and other sea ducks of the north
Pacific region as well as the diminutive cackling Canada goose of
the Yukon River Delta use the coastal sea route for all or most of
their southward flight. The journey of the cackling goose, as shown
by return records from birds banded at Hooper Bay, Alaska, has been
traced southward across the Alaskan Peninsula and apparently across
the Gulf of Alaska to the Queen Charlotte Islands. The birds then
follow the coast line south to near the mouth of the Columbia River,
where the route swings toward the interior for a short distance
before continuing south by way of the Willamette River Valley. The
winter quarters of the cackling goose are chiefly in the vicinity
of Tule Lake, on the Oregon-California line, and in the Sacramento
Valley of California, although a few push on to the San Joaquin
Valley.
A tributary of this "flyway" is followed by Ross' goose, which breeds
in the Perry River district south of Queen Maud Gulf and other areas
farther east on the central Arctic coast of Canada (Fig. 21). Its
fall migration is southwest and south across the barren grounds to
Great Slave and Athabaska Lakes, where it joins thousands of other
waterfowl bound for winter homes along the eastern coast of the
United States and the Gulf of Mexico. But when Ross' geese have
traveled south approximately to the northern boundary of Montana,
most of them separate from their companions and turn southwest across
the Rocky Mountains to winter in California. In recent years a few
Ross' geese have been found wintering east of the Rocky Mountains
along with flocks of lesser snow geese and may be correlated with an
eastward extension of their breeding range.
The southward route of those migratory landbirds of the Pacific area
that leave the United States in winter extends chiefly through the
interior of California to the mouth of the Colorado River and on to
winter quarters in western Mexico (Fig. 18, routes 6 and 7).
[Illustration: _Figure 21. The breeding range, wintering range, and
main migration route of Ross' geese. This is the only species of
which practically all members breed in the Arctic, migrate south
through the Canadian prairie, and upon reaching the United States,
turn to the southwest rather than the southeast. The southern part of
this route, however, is followed by some mallards, pintails, wigeons,
and other ducks._]
[Illustration: _Figure 22. Breeding and wintering ranges of the
western tanager. See Fig. 23 for the spring route taken by the birds
breeding in the northern part of the range._]
The movements of the western tanager show a migration route that is
in some ways remarkable. The species breeds in the mountains from the
northern part of Baja California and western Texas north to northern
British Columbia and southwestern Mackenzie. Its winter range is
in two discontinuous areas--southern Baja California and eastern
and southwestern Mexico south to Guatemala (Fig. 22). During spring
migration the birds appear first in western Texas and the southern
parts of New Mexico and Arizona about April 20 (Fig. 23). By April
30 the vanguard has advanced evenly to an approximate east-west line
across central New Mexico, Arizona, and southern California. By May
10 the easternmost birds have advanced only to southern Colorado,
while those in the far west have reached northern Washington. Ten
days later the northward advance of the species is shown as a great
curve, extending northeastward from Vancouver Island to central
Alberta and thence southeastward to northern Colorado. Since these
tanagers do not reach northern Colorado until May 20, it is evident
those present in Alberta on that date, instead of traveling northward
through the Rocky Mountains, their summer home, actually reached
there by a route that carried them west of the Rockies to southern
British Columbia and thence eastward across the still snowy northern
Rocky Mountains.
[Illustration: _Figure 23. Migration of the western tanager. The
birds that arrive in eastern Alberta by May 20 do not travel
northward along the eastern base of the Rocky Mountains, because
the vanguard has then only reached northern Colorado. Instead the
isochronal lines indicate that they migrate north through California,
Oregon, and Washington and then cross the Rockies in British
Columbia._]
-------------------------------------------------
Pacific Oceanic Route
-------------------------------------------------
The Pacific oceanic route is used by the Pacific golden plover,
bristle-thighed curlew, ruddy turnstone, wandering tattler and other
shorebirds. The ruddy turnstone, and probably other shorebirds,
migrating from the islands of the Bering Sea, have an elliptical
route that takes them southward via the islands of the central
Pacific and northward along the Asiatic coast. In addition, many
seabirds that breed on far northern and southern coasts or islands
migrate up and down the Pacific well away from land except when the
breeding season approaches.
The Pacific golden plover breeds chiefly in the Arctic coast region
of Siberia and in a more limited area on the Alaskan coast. Some of
the birds probably migrate south via the continent of Asia to winter
quarters in Japan, China, India, Australia, New Zealand, and Oceania.
Others evidently go south by way of the Pacific Ocean to the Hawaiian
Islands and other islands of the central and southern Pacific.
Migrating golden plovers have been observed at sea on a line that
apparently extends from Hawaii to the Aleutian Islands; it appears
certain some of the Alaskan birds make a nonstop flight across the
sea from Alaska to Hawaii. While it would seem incredible that any
birds could lay a course so accurately as to land on these small
isolated oceanic islands, 2,000 miles south of the Aleutians, 2,000
miles west of Baja California, and nearly 4,000 miles east of Japan,
the evidence admits only the conclusion that year after year this
transoceanic round-trip journey between Alaska and Hawaii is made by
considerable numbers of golden plovers.
-------------------------------------------------
Arctic Routes
-------------------------------------------------
Some Arctic nesting birds retreat only a short distance south in the
winter. These species include the red-legged kittiwake, Ross' gull,
emperor goose, and various eiders. The latter group of ducks winter
well south of their nesting areas but nevertheless remain farther
north than do the majority of other species of ducks. The routes
followed by these birds are chiefly parallel to the coast and may be
considered as being tributary either to the Atlantic or Pacific coast
routes. The heavy passage of gulls, ducks, black brants, and other
water birds at Point Barrow, Alaska, and other points on the Arctic
coast, has been noted by many observers. The best defined Arctic
route in North America is the one following the coast of Alaska.
A migration route, therefore, may be anything from a narrow path
closely adhering to some definite geographical feature, such as a
river valley or a coastline, to a broad boulevard that leads in the
desired direction and follows only the general trend of the land
mass. Oceanic routes appear to be special cases that are not fully
understood at the present time. Also it must be remembered that all
the main routes contain a multitude of tributary and separate minor
routes. In fact, with the entire continent of North America crossed
by migratory birds, the different groups or species frequently
follow lines that may repeatedly intersect those taken by others of
their own kind or by other species. The arterial or trunk routes,
therefore, must be considered merely as indicating paths of migration
on which the tendency to concentrate is particularly noticeable.
=PATTERNS OF MIGRATION=
Band recoveries, netting records, and personal observations help us
to critically examine migration routes and probe deeper into the
origin and evolution of these pathways. We are beginning to realize
certain deviations occur from the "normal" north and south movements
expected in most species. In the previous section on routes, we
touched briefly on the fact that some routes are not poleward at all,
but in some other direction. We know that many migrants do not stop
at the exact localities year after year but they probably do follow
the same general course each season. After many years of observations
a pattern emerges for that population, species, or group of species.
In this section we would like to take a closer look at some of
the interesting patterns (or "eccentric routes" as Cook (1915a)
referred to them) in migration that birds are annually to travel
from breeding to wintering grounds and back again. In many cases,
the causative agents are unknown or pure conjecture, but in others,
sound biological principles can be put forth that may indicate why
a particular species could have evolved the specific pattern it
exhibits.
Loops
Many species do not return north in the spring over the same route
they used in the fall; rather, they fly around an enormous loop
or ellipse. Cook (1915a) considered food as the primary factor
in determining the course birds took between winter and summer
ranges. Individuals that returned by the same route and did not find
sufficient food for their needs at that time were eliminated from the
population, and only progeny from individuals that took a different
course with sufficient food lived to build the tradition of a loop
migration. Other investigators consider prevailing winds a major
factor in the evolution of loop migration. Whatever the reason may
be, it has most likely evolved separately in each species to satisfy
its particular needs, and the fact that this pattern occurs all over
the world in completely unrelated species is a good illustration of
convergent evolution.
The annual flight of adult golden plovers is so unusual, it will be
given in some detail. The species is observed by hundreds of bird
watchers every year and it well illustrates loop migration (Fig. 24).
[Illustration: _Figure 24. Distribution and migration of the
American golden plover. Adults of the eastern subspecies migrate
across northeastern Canada and then by a nonstop flight reach South
America. In spring they return by way of the Mississippi Valley.
Their entire route, therefore, is in the form of a great ellipse with
a major axis of 8,000 miles and a minor axis of about 2,000 miles.
The western subspecies migrates across the Pacific Ocean to various
localities including the Hawaiian and Marquesas islands and the Low
Archipelago._]
In the fall, the birds fatten on the multitude of berries along
the coasts of Labrador and Nova Scotia, then depart south over the
Atlantic Ocean to South America. After reaching the South American
coast the birds make a short stop, then continue overland to the
pampas of Argentina, where they remain from September to March.
When these golden plovers leave their winter quarters they cross
northwestern South America and the Gulf of Mexico to reach the North
American mainland on the coasts of Texas and Louisiana. Thence they
proceed slowly up the Mississippi Valley and, by the early part
of June, are again on their breeding grounds, having performed a
round-trip journey in the form of an enormous ellipse with the minor
axis about 2,000 miles and the major axis 8,000 miles stretching from
the Arctic tundra to the pampas of Argentina. The older birds may
be accompanied by some of the young, but most of the immature birds
leave their natal grounds late in summer and move southward through
the interior of the country, returning in spring over essentially the
same course. The oceanic route is therefore used chiefly by adult
birds.
A return by the oceanic route in the spring could be fatal. The
maritime climate in the Northeast results in foggy conditions along
the coast and the frozen soil would offer few rewards for the weary
travelers. By traveling up the middle of the continent, a much better
food supply is assured (Welty 1962).
Several North American warblers including the Connecticut warbler
(Fig. 25) and the western race of the palm warbler have been found to
follow circuitous migration routes. The Connecticut warbler is not
observed or banded on the East coast in spring, but it is recorded
farther inland during the season. Thus this warbler proceeds down
the East coast in the fall and up the interior of the continent in
the spring. Similarly, the western race of the palm warbler moves
from its breeding grounds directly east to the Appalachian Mountains
before turning south along the coast. Television tower kills in
northern Florida indicate the population is very concentrated here
at this time of year. In the spring this race also proceeds north
through the interior. Graber (1968) points out that the eastern
race of the palm warbler also proceeds south along the coast in the
fall and poses this question: "does the western population of this
species intentionally move toward the ancestral range, or is the fall
flight direction merely a consequence of the temperate zone westerly
circulation?"
Graber concluded from radar observations that the disparity in
seasonal flight directions of many migrants was a positive response
of migrants to favorable wind directions at that time of year. The
east-oriented trans-gulf migrants followed an elliptical migration
because postfrontal air flow in the fall at latitude 40° N is
northwesterly, and, in the spring southerly; whereas winds over
the Gulf of Mexico are consistently easterly or southeasterly.
Therefore, transgulf migrants returning north in the spring would
be moved westward across the Gulf unless they compensated for wind
drift. Observers were not aware of high-altitude drift before radar
(Bellrose and Graber 1963).
[Illustration: _Figure 25. Breeding range and migration routes of
the Connecticut warbler. From the breeding range in northern United
States and southern Canada, it migrates east in the fall to New
England, then south along the Atlantic coast to Florida and across
the West Indies to winter in South America. In the spring it does not
return by the same route but rather completes a loop by migrating
northwest across the Allegheny Mountains and the Mississippi Valley
(Adapted from Cooke 1915a)._]
Numerous other loop migrations have been documented throughout the
world. In the fall, the short-tailed shearwater, is observed off
the west coast of North America as far south as California. At this
time the species is on the eastern leg of a tremendous figure-eight
circuit around the Pacific Ocean (Fig. 26). The subalpine warbler and
red-backed shrikes perform loop migrations between Europe and Africa.
Both pass much farther to the east in the spring than in the fall
(Moreau 1961). The Arctic loon travels south across inland Russia to
southern Europe but returns to its Arctic breeding grounds via the
Gulf Stream on the sea because this water is open much earlier in the
spring than the inland waterways (Welty 1962).
[Illustration: _Figure 26. Migration route of the short-tailed
shearwater. An example of an incredibly large loop migration pattern
in a pelagic species. Breeding adults return to two islands in
Bass Strait during the last part of October after completing a
figure-eight circuit of the northern Pacific Ocean (From Serventy
1953)._]
Dog-legs
Dog-leg migration patterns are characterized by a prominent bend
or twist in the route. Studies have shown some of these illogical,
out-of-the-way means for connecting wintering and breeding areas have
no biological function, but instead, are the result of tradition much
like the lineage of crooked streets in Boston can be traced back to
old cowpaths (Welty 1962). Many species have extended their range in
recent years, but the pioneers continue to retrace the old route from
the point of origin even if the new areas are not on the same axis as
the earlier route. The old pathways have apparently become implanted
as part of the migratory instinct in all members of particular
populations even after extending their ranges considerable distances
from the original.
Good examples of this crooked traditional path can be seen in the
routes taken by Old World species extending their ranges into the
New World from Europe and Asia. The European wheatear has extended
its range into Greenland and Labrador where the local breeding
population has become a separate race. When the Labrador individuals
depart from their breeding grounds, they proceed north to Greenland,
their ancestral home, then west to Europe and south to Africa, the
traditional wintering area for all wheatears. Alaskan breeding
wheatears migrate to Africa in the opposite direction via Asia where
the Alaskan population presumably originated. Alaskan breeding Arctic
and willow warblers and bluethroats also migrate westward into
Siberia and then southward on the Asiatic side. Some investigators
believe the Arctic tern colonized the New World from Europe because
when this bird departs for the south it first crosses the Atlantic
to Europe, then moves down the eastern Atlantic coast to Africa and
either back across the Atlantic to South America or continues south
down past South Africa (Fig. 11). To get to South America from the
eastern Arctic, it would be shorter to follow the golden plover's
flight path straight down the Atlantic or along the east coast of the
United States but the fact that no Arctic terns have been observed in
the Caribbean indicates that they do no follow that route.
In western United States, California gulls nest in various colonies
around Great Salt Lake and Yellowstone Park. Banding records
indicate these populations winter along the California coast (Fig.
27). Instead of traveling southwest by the shortest distance to the
wintering grounds, they proceed longitudinally down the Snake and
Columbia Rivers and reach the coast around Vancouver ( Woodbury et
al. 1946). Thence they proceed south along the coast to Oregon and
California. In the spring the adults return over the same course
rather than taking the shorter flight northeast in April across the
deserts and mountains; this route would be largely made over a cold
and inhospitable country (Oldaker 1961).
[Illustration: _Figure 27. Migration route and wintering grounds
of California gulls banded in northwestern Wyoming. During fall
migration, the birds proceed west from the breeding grounds to the
Pacific Ocean before turning south to wintering areas in California.
A more direct route across Nevada would entail a trip through
relatively barren country (After Diem and Condon 1967)._]
Sladen (1973) has mapped the migration routes of whistling swans,
and several dog-leg patterns are apparent in the eastern and western
populations (Fig. 28). In the eastern population, a sharp change in
direction occurs at their major feeding and resting areas in North
Dakota. After the birds arrive from the Arctic breeding grounds,
they proceed east-southeast to their wintering grounds on Chesapeake
Bay. In the western population, thousands of birds migrate from the
Alaskan breeding grounds to the large marshes along Great Salt Lake.
Then after a major stopover, this population heads west over the
mountains to California.
[Illustration: _Figure 28. Distribution and migration routes of
whistling swans in North America. Birds from the central arctic head
south to North Dakota before proceeding east to Chesapeake Bay, while
many Alaskan breeders migrate to Great Salt Lake before turning west
to winter in California (After Sladen, 1973)._]
Pelagic Wandering
Many of the pelagic birds observed off our coasts or at sea appear
to be nomadic when they are not breeding. These movements are not
necessarily at random because there is usually a seasonal shift in
the population, often for great distances and in specific directions,
away from the breeding area after completion of the nesting cycle.
Also the return from the sea to nesting areas is at a definite time
of year. This may not be true migration in the classical sense
(Thomson 1964), although it is similar in most respects.
Because of the extensive and often inhospitable habitat of pelagic
birds (to human observers at least), observations on their movements
are difficult at best and accurate records are few. We do know
some of these species have regular routes (e.g., Arctic terns) and
specific patterns of migration (e.g., the loop in the short-tailed
shearwater). As more knowledge is accumulated on the "nomadic"
species, we may actually find they too have regular migration routes
based on biological needs.
Movements of some of the tubenoses (Order Procellariiformes, that
includes albatrosses, fulmars, shearwaters, and petrels) have been
correlated with ocean currents, prevailing winds, temperatures, and
general water fertility (Kuroda 1957; Shuntov 1968; Fisher and Fisher
1972). Commercial fishermen have long known ocean currents are very
important factors in the supply of nutrients, plankton, and forage
fish for larger fish. These same foodstuffs often attract pelagic
birds as evidenced by the tremendous concentrations that occur off
the Peruvian coasts where the upwelling of cold nutrient-bearing
water is evident. Kuroda (1957) found some fine correlations between
the route of the short-tailed shearwater and ocean currents. Likewise
Shuntov (1968) found the migratory routes of albatrosses were over
temperate marine waters of high biological productivity. The Laysan
albatross was correlated with cold currents, while the black-footed
albatross occurred over warm currents. Many Southern Hemisphere
pelagic species have been extremely successful in exploiting rich
northern waters during the summer; the group is probably the most
abundant and widespread in the world (Bourne 1956).
Leap-frogging
When two or more races of the same species occupy different breeding
ranges on the same axis as migratory flight, the races breeding the
farthest north often winter the farthest south. Thus, a northern race
"leap-frogs" over the breeding and wintering range of the southern
populations. This has been well documented in the fox sparrow
discussed previously (Fig. 10) and is exhibited by races of Canada
geese breeding in central Canada as well. One of the smaller races of
this goose breeds along the Arctic coast of the Northwest Territories
and winters on the Gulf coast of Texas and northeastern Mexico, while
a much larger race breeds in the central United States and Canada but
winters in the central part of the United States. This leaping over
occurs in other species as well, including the bluebird (Pinkowski
1971).
Vertical Migration
In the effort to find winter quarters furnishing satisfactory living
conditions, many North American birds fly hundreds of miles across
land and sea. Others, however, are able to attain their objectives
merely by moving down the sides of a mountain. In such cases a
few hundred feet of altitude corresponds to hundreds of miles of
latitude. Movements of this kind, known as "vertical migrations," are
found worldwide wherever there are large mountain ranges. Aristotle
first mentions vertical migration: "Weakly birds in winter and in
frosty weather come down to the plains for warmth, and in summer
migrate to the hills for coolness ..." (Dorst 1962). The number of
species that can perform this type of migration pattern is obviously
limited to those species adapted to breeding in alpine areas.
In the Rocky Mountain region vertical migrations are particularly
notable. Chickadees, rosy finches, juncos, pine grosbeaks,
Williamson's sapsuckers, and western wood pewees nest at high
altitudes and move down to the lower levels to spend the winter.
The dark-eyed juncos breeding in the Great Smoky Mountains make a
vertical migration, but other members of the species, breeding in
flatter areas, make an annual north-south migration of hundreds of
miles (Van Tyne and Berger 1959). There is a distinct tendency among
the young of mountain-breeding birds to work down to the lower levels
as soon as the nesting season is over. The sudden increases among
birds in the edges of the foothills are particularly noticeable when
cold spells with snow or frost occur at the higher altitudes. In
the Dead Sea area of the Middle East, some birds that breed in this
extremely hot desert move up into the surrounding cooler hill during
the winter (Thomson 1964).
The vertical migrations of some mountain dwelling gallinaceous birds
(mountain quail and blue grouse) are quite interesting because the
annual journey from breeding to wintering grounds is made on foot.
Mountain quail make this downward trek quite early in the fall well
before any snows can prevent them from reaching their goal. Blue
grouse perform essentially the same journey in reverse. During
midwinter, these birds can be found near timberline eating spruce
buds protruding above the snow.
These illustrations show that the length and direction of a migration
route are adapted to the needs for survival and are met in some cases
by a short vertical movement or great latitudinal travels in others.
Pre-migratory Movements
Recent banding studies have demonstrated many migrants, especially
young of the year, have a tendency to disperse after fledging.
These premigratory movements have also been called "post-fledging
dispersal," "reverse migration," and "postbreeding northward
migration." Demonstration of this phenomenon is especially important
as it relates to locality-faithfulness (Ortstreue), range extension,
and gene mixture between populations. These movements cannot be
considered as true migrations even though they are repeated annually
by the species between breeding grounds and some other area. These
movements are generally repeated by the same age class in the
population but not the same individuals.
Nevertheless, these regular northward movements are quite striking,
especially in herons. The young of some species commonly wander
late in the summer and fall for several hundred miles north of the
district in which they were hatched. Young little blue herons as well
as great and snowy egrets are conspicuous in the East as far north
as New England and in the Mississippi Valley to southeastern Kansas
and Illinois. Black-crowned night herons banded in a large colony
at Barnstable, Massachusetts, have been recaptured the same season
northward to Maine and Quebec and westward to New York. In September
most of them return to the south.
These movements have been noted in several other species as well.
Broley (1947) nicely illustrated this northward movement of bald
eagles along the Atlantic coast (Fig. 29). Birds banded as nestlings
in Florida have been retaken that summer 1,500 miles away in Canada.
Van Tyne and Berger (1959) surmised the summer heat of Florida was
too great for this eagle, a northern species that has only recently
spread into Florida to take advantage of abundant food and nesting
sites, which it exploits during the cooler season. Postbreeding
northward movements are also shared by wood ducks, yellow-breasted
chats, eastern bluebirds, and white pelicans.
A somewhat different type of postbreeding migration is the so-called
"molt migration" exhibited by many species of waterfowl (Salomonsen
1968). These birds may travel considerable distances away from their
nesting area to traditional molting sites where they spend the
flightless period of the eclipse plumage. At such times they may move
well into the breeding ranges of other geographic races of their
species. These movements may be governed by the availability of food
and are counteracted in fall by a directive migratory impulse that
carries those birds that attained more northern latitudes after the
nesting period, back to their normal wintering homes in the south.
Vagrant Migration
The occasional great invasions beyond the limits of their normal
range of certain birds associated with the far North are quite
different from migration patterns discussed previously. Classic
examples of such invasions in the eastern part of the country are
the periodic flights of crossbills. Sometimes these migrations will
extend well south into the southern States.
[Illustration: _Figure 29. Northern recoveries of young bald eagles
banded as nestlings in Florida. The birds sometimes "migrate" over
1,500 miles up the coast during their first summer before returning
south (From Broley 1947)._]
Snowy owls are noted for occasional invasions that have been
correlated with periodic declines in lemmings, a primary food
resource of northern predators. According to Gross (1947), 24 major
invasions occurred between 1833 and 1945. The interval between these
varied from 2 to 14 years, but nearly half (11) were at intervals
of 4 years. A great flight occurred in the winter of 1926-27 when
more than 1,000 records were received from New England alone, but
the largest on record was in 1945-46 when the "Snowy Owl Committee"
of the American Ornithologists' Union received reports of 13,502
birds, of which 4,443 were reported killed. It extended over the
entire width of the continent from Washington and British Columbia
to the Atlantic coast and south to Nebraska, Illinois, Indiana,
Pennsylvania, and Maryland. One was taken as far south as South
Carolina.
In the Rocky Mountain region, great flights of the beautiful Bohemian
waxwing are occasionally recorded. The greatest invasion in the
history of Colorado ornithology occurred in February 1917, when it
was estimated that at least 10,000 were within the corporate limits
of the city of Denver. The last previous occurrence of the species in
large numbers in that section was in 1908.
Evening grosbeaks likewise are given to more or less wandering
journeys, and, curiously enough, in addition to occasional trips
south of their regular range, they travel east and west for
considerable distances. For example, grosbeaks banded at Sault Ste.
Marie, Michigan, have been recaptured on Cape Cod, Massachusetts, and
in the following season were back at the banding station. Banding
records and museum specimen identifications demonstrate that this
east-and-west trip across the northeastern part of the country is
sometimes made also by purple finches, red crossbills, and mourning
doves.
=ORIGIN AND EVOLUTION OF MIGRATION=
The origin and evolution of bird migration has been discussed in
ornithological literature for centuries. As we have seen from the
foregoing discussion, migration exists in many forms throughout the
world and probably arose to satisfy many different needs in different
orders of birds at the same time. New pattens, traditions, and routes
are arising today as well as disappearing. Currently, the migration
patterns we see are a composite result of historic influences mixed
with present day influences. Even though the migration of several
different species may be very similar, the patterns exhibited today
can be the result of quite different evolutionary processes. Because
it cannot be substantiated by experimental facts, any explanation of
how a particular pattern or route originates is pure conjecture.
The general anatomical and physiological attributes of birds enable
them to develop more diverse and spectacular migratory behavior
than any other group of animals. Their potential for long sustained
flights is of primary importance in pre-adapting birds to successful
migrations. Migration has long since become a definite hereditary
habit of many species of birds that recurs in annual cycles,
evidently because of physiological changes which prompt a search
for an environment suitable for reproduction and survival. Like the
bird's other habits its migratory behavior is just as characteristic
as the color of its plumage and, like it, evolved through natural
selection because it was advantageous for the survival of the
population. Its origin has been thought by some to be a mystery
locked in past ages, but by study of the history of how birds came
to occupy their present ranges, information becomes available which
suggests theories that may be developed and explored. Two that are
commonly mentioned are termed the "Northern Ancestral Home Theory"
and the "Southern Ancestral Home Theory."
According to the former of these hypotheses, in earlier ages when
conditions of climate, food, and habitat were favorable for existence
of birds throughout the year much further north than is the case
today, many species remained in these northern latitudes as permanent
residents. Today, such conditions are found only in more southern
regions where migrations are much shorter or nonexistent. Gradually,
however, in the Northern Hemisphere the glacial ice fields advanced
southward, causing a southward movement of conditions favorable to
northern birds, until finally all bird life was confined to southern
latitudes. As the ages passed, the ice cap gradually retreated, and
each spring the birds whose ancestral home had been in the North
moved in again to fill newly opened breeding habitat only to be
driven south again at the approach of winter. As the size of the
ice-covered area diminished, the journeys to the summer breeding
areas became even longer until eventually the climatic conditions of
the present age became established, and with them, present patterns
of the annual advance and retreat we call migration.
The opposing theory is simpler in some respects and supposes the
ancestral home of Northern Hemisphere migratory birds was in the
Tropics. As all bird life tends to overpopulation, there was a
constant effort of young individuals to pioneer and seek breeding
grounds where competition was less severe. Species better adapted to
more northern latitudes moved in that direction for nesting but were
kept in check by the glacial ice and forced to return southward with
the recurrence of winter conditions. Gradually, as the ice retreated,
vast areas of virgin country became successively suitable for summer
occupancy, but the winter habitat in the South remained the home to
which the birds returned after the nesting season.
The above two theories presume that the Quaternary glaciations, which
occurred 10,000 to 1 million years ago, have been the predominate
influence on bird migration in North America and Europe as we observe
it today. There is no doubt these extreme climatic and ecologic
barriers played a part in shaping or modifying some patterns, but
as Moreau (1951) has pointed out, well-developed migrations occur
in parts of the world, including the Southern Hemisphere or even
within the tropics, where continental glaciation was not a factor.
Furthermore, migrations to fit various needs have probably been
going on ever since birds could fly. The tremendous Pleistocene
glaciations actually occupied less than a hundredth of the time birds
have existed on the earth and probably only determined the details of
migrations as we see them today (Moreau 1951).
The northern and southern ancestral home theories appear
diametrically opposed to each other but Dorst (1963) concludes they
are perfectly compatible. Since the phenomena probably occurred
simultaneously, northern migrants then originated from two stocks:
the North Temperate Zone birds sought refuge to the south during
the glacial periods and the tropical avifauna expanded their range
during the interglacial periods. Dorst also stated this double origin
is more prevalent in North America where the tropical element is
most abundant. Birds representing this element include hummingbirds,
tyrant flycatchers, orioles, tanagers, and blackbirds. At some
latitudes, they nest in the same area as the shorebirds which are of
arctic parental stock.
These theories assume migration is a genetic, inherited character,
but we now know in some species it can be modified in the lifetime
of one individual and the place some individuals return to nest
or winter is not the ancestral home but a place to which they had
been transported at an early stage in their development. Traditions
that have lived for countless generations may die overnight if
experienced individuals are lost or no longer active (Hochbaum 1955);
migration patterns remain constant only as long as the factors
influencing these patterns remain constant. But the landscape and
the interacting ecological stresses are forever changing, and we
would expect the adaptive behavior of birds to respond with them.
One of these responses to an expanding habitat is colonization of
new territory and expansion of a species' range with accompanying
development of a migratory habit. The search for favorable
conditions under which to breed in summer and to feed in winter, as
influenced by competition for space, has been the principal factor
underlying the extension of ranges, usually by young, nonconditioned
individuals. This is exemplified by the northward extension in
historic times of a number of species. Many of these range extensions
have closely followed man's settlement of the area and the subsequent
changes in habitat that man has made.
From the previous descriptions of migration patterns and routes,
it will be observed that the general trend of migration in most
northern populations of North American birds is northwest and
southeast. Eastern species tend to extend their ranges by pushing
westward, particularly in the North. For example, in the Stikine
River Valley of northern British Columbia and southwestern Alaska the
common nighthawk, chipping sparrow, rusty blackbird, yellow warbler,
American redstart, and others have established breeding stations at
points 20 to 100 miles from the Pacific Ocean. The northern race of
the American robin, common flickers, dark-eyed juncos, blackpoll
warblers, yellow-rumped warblers, and ovenbirds, all common eastern
species, also are established as breeding birds in western Alaska.
The ovenbird has even been detected on the lower Yukon River, and
the sandhill crane and gray-cheeked thrush have moved across Bering
Strait into Siberia. These birds continue to migrate through the
eastern part of the continent. Instead of taking the shortest route
south, they retrace the direction of their westward expansion and
move southward along the same avenues as their more eastern relatives.
The red-eyed vireo is essentially an inhabitant of states east of
the Great Plains, but an arm of its breeding range extends northwest
to the Pacific coast in British Columbia (Fig. 30). It seems evident
this is a range extension that has taken place comparatively recently
by a westward movement via deciduous woodland corridors, and the
invaders retrace in spring and fall the general route by which they
originally entered the country.
In the case of the bobolink, a new extension of the breeding
range and a subsequent change in the migration of the species has
taken place since settlement by European man (Fig. 19). Because
the bobolink is a bird of damp meadows, it was originally cut off
from the Western States by the intervening arid regions, but with
the advent of irrigation and the bringing of large areas under
cultivation, small colonies of nesting bobolinks appeared at various
western points. Now the species is established as a regular breeder
in the great mountain parks and irrigated valleys of Colorado and
elsewhere almost to the Pacific coast. These western pioneers must
fly long distances east and west to reach the western edge of the
route followed by the bulk of the bobolinks that breed in the
northern United States and southern Canada.
[Illustration: _Figure 30. Distribution and migration of the red-eyed
vireo. It is evident that the red-eyed vireo has only recently
invaded Washington by an extension of its breeding range almost due
west from the upper Missouri Valley. Like the bobolink (Fig. 19),
however, the western breeders do not take the short cut south or
southeast from their nesting grounds but migrate spring and fall
along the route traveled in making the extension._]
During the past few decades, various populations of Canada geese have
altered their migration patterns as a result of transplanting brood
stock, development of refuges or changing agricultural practices.
These routes will continue to change in the coming years as long as
these factors are in a state of flux. It has been shown that man can
establish breeding colonies of Canada geese with young birds almost
anywhere.
Europe also has several good examples of changes in migration routes
through range extension. One of the best examples is the serin.
During the past century, this European finch has spread its breeding
range from around the Mediterranean Sea to include the entire
continent. While the Mediterranean populations remain sedentary,
the more northern breeding birds are migratory. Most likely, those
birds that did not migrate from the North were eliminated by severe
weather. Similarly, the wheatear, yellow wagtail, and Arctic warbler
have extended their breeding ranges eastward across the Bering Sea
into Alaska, but the wheatear, for instance, migrates all the way
back across Asia to Africa where it winters with other wheatears
coming from Europe, Iceland, and Greenland.
As bird populations become more and more migratory, we might expect
their flight capabilities to be enhanced accordingly. These changes
in morphology are readily seen in wing shape. Several groups of
birds have closely related species or populations some of which are
migratory and others sedentary. The sedentary species or populations
have more rounded wings because of the relative length of the wing
quills. On the other hand, populations that migrate great distances,
such as albatrosses, falcons, swifts, various shorebirds, and
terns, have more pointed wings. Kipp(1942, 1958) demonstrated this
using orioles. The sedentary black-headed oriole of India has a
well-rounded wing whereas the closely related black-naped oriole is
migratory between India and Siberia and has primaries that are much
more pointed and well developed.
Thus it seems the origin and evolution of migration have roots in the
present that are deep in the past. The important thing to consider
in the evolution of a migratory trait is whether a population can
adapt to new conditions by genetic modification of its physiology
and habits. The migratory habit has evolved in those populations
in which, on the average, more individuals survive by moving to a
different area part of the year than if they remained in the same
area all year.
=WHERE WE STAND=
The migration of birds had its beginning in times so remote its
origins have been largely obscured and can be interpreted now only
in terms of present conditions. The causes underlying migration are
exceedingly complex. The mystery that formerly cloaked the periodic
travels of birds, however, has been largely dispelled through the
fairly complete information now available concerning the extent and
times of seasonal journeys of most species. Many gaps still remain
in our knowledge of the subject, but present knowledge is being
placed on record, and the answers to many uncertainties that continue
to make bird migration one of the most fascinating subjects in the
science of ornithology must be left for future studies. In some areas
we are on the threshold of discovery. More and more sophisticated
approaches including radar, radio telemetry, computer processing
of banding data, and physiological and behavior studies are being
developed.
With the widespread use of these new techniques, we are beginning
to realize the benefits, aside from aesthetic reasons, for studying
migration. Radar alone has aided tremendously in documenting flock
size, heights, and speeds of migration as well as the descriptions
and locations of patterns and routes of specific migrants in relation
to aircraft flight lanes. Recent studies have indicated local,
nonmigratory populations of various blackbirds cause nearly all of
the rice damage in southern States and the "hordes from the North"
contribute very little to the losses. In addition, the transport of
arborviruses from one continent to another via these long distance
migrants is being investigated. People have started to uncover the
secrets of migration and utilize this knowledge for the betterment of
our society.
Each kind of bird seems to have its own reaction to the environment,
so that the character of movement differs widely in the various
species, and seldom do any two present the same picture. In
fact, bird migration has been described as a phase of geographic
distribution wherein there is a more or less regular seasonal
shifting of the avian population caused by the same factors that
determine the ranges of the sedentary species. If this view is
correct, then it must be recognized that the far-reaching works of
man in altering the natural condition of the Earth's surface can so
change the environment necessary for the well-being of the birds as
to bring about changes in their yearly travels. The nature and extent
of the changes wrought by man on the North American Continent are
readily apparent. Extensive forests have been burned or cut away,
rolling prairies turned over with the plow, and wetlands drained
or filled. Their places have been taken by a variety of human
activities. These great changes are exerting pressure on native bird
populations, and various species may be either benefited or adversely
affected.
The Federal Government has recognized its responsibility to migratory
birds under these changing conditions. Enabling acts allow for
carrying out migratory bird treaty obligations in cooperation with
other countries, and now most species have legal protection under
regulations administered by the U.S. Fish and Wildlife Service. The
effectiveness of conservation laws, however, is increased in the
same measure that the people of the country become acquainted with
the migratory bird resource and interest themselves personally in
the well-being of the various species. Long before European man
came to America, the birds had established their seasonal patterns
of migration throughout the Western Hemisphere. The economic,
scientific, and esthetic values of these migratory species dictate
they be permitted to continue their long-accustomed and to some
extent still-mysterious habits of migration.
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