Transcriber's Note
Emphasized text is denoted by _Italics_ and =Bold=.
[Illustration]
MIGRATION of BIRDS
[Illustration]
CIRCULAR 16
FISH AND WILDLIFE SERVICE
UNITED STATES DEPARTMENT OF THE INTERIOR
MIGRATION of BIRDS
[Illustration]
By FREDERICK C. LINCOLN
_Biologist_
Illustrated by BOB MINES
CIRCULAR 16
UNITED STATES DEPARTMENT OF THE INTERIOR, Oscar L. Chapman, _Secretary_
FISH AND WILDLIFE SERVICE, Albert M. Day, _Director_
[Illustration]
UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1950
For sale by the Superintendent of Documents, Washington 25, D. C.
Price 30 cents
Contents
Page
The mystery of migration 4
Advantages of migration 6
The origin of migration 7
Northern ancestral home theory 8
Southern ancestral home theory 8
Theory of photoperiodism 9
Theory of continental drift 11
When birds migrate 11
Movements of species and groups 12
Nocturnal and diurnal migration 15
How birds migrate 18
Speed of flight and speed of migration 18
Altitude at which birds travel 26
Orientation 28
Segregation during migration 31
Where birds migrate 34
Short and undetermined migrations 34
Variable migrations within species 35
Fall flights not far south of breeding ranges 37
Long-distance migrations 38
Routes of migration 41
Wide and narrow migration lanes 42
The fly ways 46
Atlantic oceanic route 48
Atlantic coast route and tributaries 53
Mackenzie Valley--Great Lakes--Mississippi Valley Route and
tributaries 60
Great Plains--Rocky Mountain routes 62
Pacific coast route 63
Pacific oceanic route 66
Arctic routes 68
Evolution of migration routes 70
Vertical migration 72
Vagrant migration 73
Perils of migration 75
Storms 75
Aerial obstructions 76
Exhaustion 78
Influence of the weather on migration 80
Problems of migration 81
Banding studies 81
Movements of residents 82
Migration of the white-throated sparrow 84
Migration of the yellow-billed loon 84
Conclusions 85
Appendix I--List of birds mentioned in the text 87
Appendix II--Bird banding 92
Bibliography 94
Index 99
MIGRATION of BIRDS
When the birds that have nested in our dooryards and those that have
frequented the neighboring woods, hills, and marshes leave us in the
fall, the question naturally comes to mind: Where do they go? This,
however, is only one small part of the question as we also wonder:
Will the same ones return next spring to their former haunts? What
dangers will they face on their round-trip flight and while in their
winter homes? These and other questions on the migratory habits
of most species of Northern Hemisphere birds puzzle all who are
interested in them, whether it be the farmer who profits by their
tireless warfare against the weed and insect pests of his crops,
the bird student who enjoys an abundance and variety of feathered
inhabitants about him, or the hunter who wants a continuation from
year to year of the sport of wildfowling. Lack of information on the
subject may mean the loss of an important resource by unconsciously
letting it slip from us, as ignorance might be responsible for
inadequate legal protection for such species as might urgently
need it. More general knowledge on the subject will aid in the
perpetuation of the various migrants, the seasonal habitats of some
of which are in grave danger from man's utilization, sometimes
unwisely, of the marsh, water and other areas that were formerly
homes for birds.
The migrations of birds were probably among the first natural
phenomena to attract the attention and intrigue 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?" Jeremiah (8:7), wrote: "The stork in the heavens
knoweth her appointed time; and the turtle [dove], and the crane, and
the swallow, observe the time of their coming." And the flight of
quail that saved the Israelites from starvation in their wanderings
in the wilderness of Sinai is now recognized as a vast movement of
migratory quail between their breeding grounds and their winter home
in Africa.
Throughout the ages the return flights of 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, and
among the Eskimos and other tribes the phenomenon to this day is
the accepted sign of the imminence of spring and of warmer weather.
The pioneer fur traders in Alaska and Canada offered rewards to the
Indian or Eskimo who saw the first goose of the spring, and all
joined in jubilant welcome to the newcomer.
As the North American Continent became more thickly settled, the
large flocks of ducks and geese 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 weed
and insect pests. The need for laws protecting the valuable game
and nongame birds and for regulating the hunting of the 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 food habits of the various species, their
environmental needs, and their travels. Hence bird investigations are
made by the Fish and Wildlife Service, the bureau 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 Canada, and between the United States and Mexico.
For more than half a century the Fish and Wildlife Service and
its predecessor, the Biological Survey, have been collecting data
on the interesting and important phenomenon of the migration of
North American birds. The field men of the Service have gathered
information concerning the distribution and seasonal movements of
the different species in many extended areas, from the Arctic coast
south to the pampas of Argentina. Supplementing these investigations
is the work of hundreds of volunteer ornithologists and bird students
throughout the United States and Canada, who each year, spring and
fall, forward to the Service reports on the migrations as observed in
their respective localities. Added to the mass of data thus assembled
is a rapidly growing recovery file of marked individuals. These
data, together with other carded records gleaned by the Fish and
Wildlife Service from a vast literature, constitute a series of files
that now contain well over 3,000,000 entries, easily the greatest
existing accumulation of information pertaining to the distribution
and movements of North American birds. Not only do the facts thus
assembled form the basis of regulatory action for the protection
of the birds, but they also make it possible to publish scientific
accounts of the ranges and migrations of the different species. They
furnish the basis of this bulletin.
The several important bird-protective measures adopted by State and
Federal Governments, particularly those having as their objectives
the conservation of the migratory song, insectivorous, and game
species, can be effective only if they have intelligent public
support. To increase such support, information must be more generally
available on that little understood but universally fascinating
subject of bird migration. A brief presentation of facts on the
migratory habits of the birds scientifically gathered by the Fish
and Wildlife Service over many years, will be helpful to bird-study
classes, to conservation organizations, and to farmers and others
individually interested in the welfare of the birds.
In addition to his original investigations in the field and in the
files of the Fish and Wildlife Service, the author has made free use
of the writings of many other students of the subject. To all of
these grateful acknowledgment is made.
_The Mystery of Migration_
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 that cranes traveled from the
steppes of Scythia to the marshes at the headwaters of the Nile,
and that 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, the elder Pliny, 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 the European blackbird, the starling, and
the thrushes.
In spite of the keen perception shown in some of his statements
Aristotle also must be credited with the origin of some superstitious
beliefs that persisted for several centuries. One of these, that
of hibernation, became so firmly rooted that Dr. Elliott Coues
(1878),[1] one of America's greatest ornithologists, listed the
titles of no less than 182 papers dealing with the hibernation
of swallows. The hibernation theory accounted for the autumnal
disappearance of certain species of birds by having them pass into
a torpid state and so remain 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 that allegedly were seen congregating in the
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.
[1] Publications referred to parenthetically by date are listed
in the Bibliography, p. 94.
The hibernation theory survived for more than 2,000 years and, until
the winter home of the chimney swift was discovered in 1944 through
the recovery of banded individuals, it was occasionally repeated
by credulous persons to account for the sudden disappearance of
the immense flocks that each autumn gather in southern Georgia and
northern Florida. Although the winter range is still unknown in
fullest detail, Lincoln (1944b) has shown that some of these birds
spend the winter season in northeastern Peru.
Although the idea that hibernation is a regular feature of the life
cycle of birds is no longer accepted for any species, recognition
must be accorded the observations of Edmund C. Jaeger of Riverside
College, Riverside, Calif. (1949). Earlier (1948), he had given a
brief account of the behavior of a poor-will found during the winter
of 1946-47 in the Chukawalla Mountains of the Colorado Desert,
Calif., and which was in a state of profound torpidity.
What was presumably the same individual was found in the same rock
niche in a comatose condition on November 26, 1947. Beginning on
December 30, 1947, rectal temperatures were taken every 2 weeks, the
last on February 14, 1948. The temperature dropped from 67.6° on the
first date to 64.4° on January 18 and February 1, recovering to 65.8°
on the late date of record. The weight decreased from 45.61 grams on
January 4 to 44.56 grams on February 14. An attempt to detect heart
beat by the use of a medical stethoscope was negative. No movement of
the chest walls could be detected and no moisture could be collected
on a cold mirror placed in front of the nostrils. Strong light aimed
directly into the pupil resulted in no response, not even an attempt
to close the eyelid. No waste matter was passed during the entire
period of observation and all evidence indicated that the bird was in
an exceedingly low state of metabolism.
This bird was banded on January 5, 1948, with a Service band and
was back in the same rock niche on November 24, 1948, certainly
the second and probably the third season of return to this exact
point. It was there on December 5, 1948, but 2 weeks later it had
disappeared, probably the victim of some predator or an inquisitive
human. Professor Jaeger reports that the Hopi Indians call the
poor-will "Holchko," the sleeping one.
Aristotle also was the originator of the theory of transmutation,
basing it upon the fact that frequently one species will arrive
from the north just as another species departs for more southerly
latitudes. From this he reasoned that although it was commonly
believed that such birds were of two different species, there really
was only one, and that this one assumed the different plumages to
correspond with the summer and winter seasons.
Probably the most remarkable theory that has been advanced to account
for migration is contained in a pamphlet mentioned by Clarke (1912:
V. I, 9-11) as published in 1703 under the title: "An Essay Toward
the Probable Solution of this Question: Whence come the Stork and
the Turtle, the Crane, and the Swallow, when they Know and Observe
the Appointed Time of their Coming." It was written "By a Person of
Learning and Piety," whose "probable solution" was that migratory
birds flew to the moon and there spent the winter.
Some peoples, who easily accepted the migratory travels of the larger
birds, were unable to understand how the smaller species, some of
them notoriously poor fliers, could make similar journeys. They
accordingly conceived the idea that the larger species, as the storks
and cranes, carried their smaller companions as living freight. In
some of the Mediterranean countries, it is still believed that these
broad-pinioned birds serve as aerial transports for the hosts of
small birds that congregate upon the shores awaiting opportunity for
this kind of passage to their winter homes in Africa. Similar beliefs
have been found among some tribes of North American Indians.
_Advantages of Migration_
Before presenting some of the present theories concerning the origin
of bird migration, it seems well to consider briefly the ends that
are served by this annual round trip between breeding grounds and
winter quarters. It is apparent that the migratory habit enables a
species to enjoy the summers of northern latitudes while avoiding the
severity of the winters. In other words, migration makes it possible
for some species to inhabit two different areas during the seasons
when each presents favorable conditions. In the performance of its
reproductive duties, every pair of birds requires a certain amount of
territory, the extent of which varies greatly in different species.
Generally, however, this territory must be large enough to provide
adequate food, not only for the parent birds, but also for the lusty
appetites that come into being with the hatching of the eggs. Thus,
if all birds were to remain constantly either in tropical or in
temperate regions, there would be intolerable overcrowding during the
breeding season. By the spring withdrawal to regions uninhabitable
earlier in the year, the migrants are assured of adequate space and
ample food upon their arrival in the winter-freed North, and it may
be assumed that the nonmigratory species that are resident in the
South are also benefited by the departure of the migrants.
Nevertheless, it cannot be said that the winter or summer area of
every species is entirely unsuited to its requirements at other
seasons, for some individuals pass the winter season in areas that
are frequented only in summer by other individuals of their species.
Such species have extensive breeding ranges, presenting wide climatic
variations, so that some individuals may actually be resident in a
region where others of their kind are present only in winter.
The tendency of some birds to move southward at the approach of
winter is not always due to the seasonal low temperatures, since
experiments have demonstrated that many of our summer insect feeders,
when confined in outdoor aviaries, comfortably withstand temperatures
far below zero. The main consideration is the depletion of the food
supply, caused either by the disappearance or the hibernation of
insects, or by the mantle of snow or ice that prevents access to the
seeds and other forms of food found on or close to the ground or
submerged in water. Possibly also the shortened hours of daylight
materially restrict the ability of the birds to obtain sufficient
food at a time when the cold requires an increased supply to maintain
body heat. It is noteworthy that some of our smaller birds, such
as the chickadees, have no fear of Arctic weather, as their food
supplies are entirely arboreal and so are always available. Also,
when there is a good supply of food in the form of pine and spruce
seeds, nuthatches and crossbills will remain through the winter in
Canadian woods. When these birds appear abundantly in winter at
points in southern latitudes, it may be concluded that there is a
shortage of their food in the North.
_The Origin of Migration_
Migration has long since become a definite hereditary habit that
recurs in annual cycles, probably because of physiological stimulus
associated with the reproductive period. Its origin is locked in the
ages of geologic time, but by study of the history of how birds came
to occupy their present ranges, information becomes available from
which reasonable theories may be developed and explored. The two that
are most commonly accepted are diametrically opposed to each other.
=Northern ancestral home theory=
According to one of these hypotheses, in earlier ages nonmigratory
birds swarmed over the entire Northern Hemisphere. At that time the
conditions of food and habitat were such as to permit them to remain
in their haunts throughout the year, that is, the entire northern
area then afforded the two important avian requirements--suitable
breeding conditions, and year-long food supply. This is the condition
today in the Tropics, and it is noteworthy that, as a rule, tropical
birds do not perform migrations. Gradually, however, in the Northern
Hemisphere the glacial ice fields advanced southward, forcing the
birds before them, until finally all bird life was concentrated
in southern latitudes. As the ages passed the ice cap gradually
retreated, and each spring the birds whose ancestral home had been in
the North endeavored to return, only to be driven south again at the
approach of winter. As the size of the ice-covered area diminished
the journeys made became ever longer until eventually the climatic
conditions of the present age became established and with them the
habit of migration.
Thus, this theory supposes that today migratory birds follow the path
of a great racial movement that took place in a distant past and was
associated with the advances and recessions of the ice. The actions
of the birds themselves lend some support to this theory, as every
bird student has noted the feverish impatience with which certain
species push northward in spring, sometimes advancing so rapidly upon
the heels of winter that they perish in great numbers when overtaken
by late storms. It is probable that at this season the reproductive
impulse is a determining factor in driving the birds to their
northern breeding grounds.
=Southern ancestral home theory=
The opposing theory is simpler in some respects and supposes that the
ancestral home of all birds was in the Tropics and that, as all bird
life tends to overpopulation, there was a constant effort to seek
breeding grounds where the competition would be less keen. Species
that strove for more northern latitudes were kept in check by the ice
and were 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 remained the home to which the birds returned after
the nesting season. It is a fact that some species spend very little
time on their breeding grounds; the orchard oriole, for example,
spends only 2½ months in its summer home, arriving in southern
Pennsylvania about the first week in May and leaving by the middle of
July.
Both of these theories assume that migration is an ingrained habit,
but both have been criticized on biological and geological grounds,
so neither should be accepted without qualification as definitely
accounting for the origin of bird migration. It is apparent, however,
that whether the ancestral home of any species was at the northern or
southern limits of its present range, or even in some intermediate
region, the search for favorable conditions under which to breed in
summer and to feed in winter has been the principal factor underlying
the origin of migration.
=Theory of photoperiodism=
A modern view based on studies of living behavior, suggests also that
there is good reason for believing that migration is an annually
induced movement. If such be true then the theory of photoperiodism
as propounded by some recent investigators should receive some
consideration.
This theory holds as its major premise that quantity of light and
length of day are the stimulating causes of migration. Its proponents
urge that migration is a phenomenon far too regular to be created
anew each season merely under stress of circumstances, such as need
for food; and that it begins before the necessity for a change in
latitude becomes at all pressing. Swallows, nighthawks, shore birds,
and others may start their southward movement while the summer food
supply in the North is at peak abundance; while robins, bluebirds,
and others may leave an abundant food in the South in spring and
press toward northern points when food supplies there are almost
entirely lacking and when severe cold and storms are likely to cause
their wholesale destruction. The regularity of arrival and departure
is one of the most impressive features of migration, and since birds
travel in almost strict accordance with the calendar, the proponents
of the theory ask: "What phenomenon to which we may attribute the
stimulating impulse occurs with such precise regularity as the
constantly increasing amount of light in spring?"
Experimental work has abundantly demonstrated the effect of increased
light upon the growth, flowering, and fruiting of plants. Similarly,
experiments with the common junco or snowbird reported by Rowan (1931:
121), resulted in increased development of the sexual organs by
the end of December, although the birds were confined in outdoor
aviaries in Canada, and had been exposed to temperatures as low as
-44° F. From the first of November until early in January, the juncos
were subjected to ever-increasing light, supplied in the aviaries by
electric bulbs. As regards illumination, they were thus artificially
provided with conditions approximating those of spring. At the close
of this period, it was found that the sexual organs of the birds had
attained the maximum development normally associated with spring.
With gradual reduction of the lighting over a period of little more
than 1 month, the organs returned to their normal winter condition.
After a consideration of all evidence, including the fact that no
ultra-violet rays were used, it was concluded that the explanation
lay in the increased exercise taken during the periods of increased
light. A simple test whereby certain birds were forced by mechanical
means to take more exercise, the light being so reduced that there
was merely sufficient glow for them to see the advancing mechanism
that forced them into movement, showed that the rate of development
of the sexual organs exactly paralleled that in the birds that were
exposed to extended periods of illumination in the outdoor aviaries.
Other features in this experiment--such as the behavior of the birds
themselves--also indicated that more activity due to increased light
is the governing cause of the spring development of the sexual
organs. If this development be accepted as a controlling cause of
migration, then this experiment must be recognized as of great
importance.
Upon closer analysis, however, it is found that this theory, like
those before discussed, is open to serious objections. First, some of
our summer residents that migrate south for the winter do not stop
in equatorial regions, where they might find the periods of day and
night about equally divided, but push on beyond, some penetrating as
far south as Patagonia. Also it might be asked: "If the lengthening
day is the stimulating factor, why should our summer birds wintering
in the Tropics ever start northward, as in their winter quarters
the variation in the length of day from winter to summer is
imperceptible?" Like all the other theories advanced, this also, as
at present understood, is subject to unanswered criticism.
=Theory of continental drift=
The theory of continental drift postulates an original northern land
mass, called Laurasia, and a southern one, called Gondwana. According
to this concept, each eventually broke into several segments which
eventually became the present continents. It is further assumed that
occasionally Laurasia and Gondwana drifted close to one another or
were at times in actual contact. On the basis of this geological
theory, Wolfson (1940) has attempted to explain the migrations of
some species of birds from one hemisphere to the other, as, for
example, the Greenland wheatear, Arctic tern, and several shore
birds (turnstone, sanderling, knot, golden plover, and others).
Acceptance of this hypothesis requires abandonment of the belief that
the development of migration was the result of useful ends that were
served thereby, and in its place, to give approval to the idea that
migration was merely "the natural consequence of an inherent behavior
pattern responding to the drifting of continental masses."
It is a strange fact that although almost all professional
paleontologists are agreed that existing data oppose the theory of
continental drift, those who support it contend that their case is
strengthened by these same data. If, in the geologic history of the
earth, there was any such thing as continental drift, it appears
from the evidence available that it was before the Cretaceous
period, estimated to have been about 70,000,000 years ago. Birds had
then evolved but those known from fossil remains were of extremely
primitive types such as Hesperornis and Ichthyornis. There is no
evidence of the existence in that period of any birds that were
even closely related to any of those now living. Accordingly, it is
difficult to believe that the migratory patterns of existing species
have been determined by events that, if they did take place, were at
least 70,000,000 or more years ago.
_When Birds Migrate_
It is known that at any given point many species leave in the fall
and return in the spring. Since banding has had such wide application
as a method of study, it is known also that in some species one of
the parent birds (rarely both) frequently returns and nests in the
tree, bush, or box that held its nest in the previous season. One
ordinarily thinks of the world of birds as quiescent during two
periods each year, at nesting time, and in winter. For individuals
this is obviously the case, but when the entire avifauna of the
continent is considered it is found that there are at almost all
periods some latitudinal movements.
=Movements of species and groups=
Some species begin their fall migrations early in July and in some
parts of the country distinct southward movements can be detected
from then until the beginning or middle of winter. For example,
many shore birds start south in the early part of July, while the
goshawks, snowy owls, redpolls, Bohemian waxwings, and many others
do not leave the North until forced to do so by the advent of severe
winter weather, or by lack of the 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 arrivals, the northern
movement may continue into the first week of 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
while others of that species are still on their way north.
A study of these facts indicates that sometimes there exists a very
definite relationship between what we may term northern and southern
groups of individuals of the same species. A supposition is that
for a species with an extensive latitudinal breeding range, and
which has a normal migration, those individuals that nest farthest
south migrate first and proceed to the southern part of the winter
range; those that occupy the central parts of the breeding range
migrate next and travel to regions in the winter range north of
those occupied by the first group; and finally the individuals
breeding farthest north are the last to start their autumn migration
and they remain farthest north during the winter. In other words,
this theory supposes that the southward movement of the species is
such that the different groups maintain their relative latitudinal
position with each other. The black and white warbler furnishes
an example. The breeding range of this bird extends west and
northwest from northern Georgia and South Carolina to New Brunswick,
extending also in a western and northwestern direction as far as
Great Bear Lake in northwestern Canada (fig. 1). It spends the
winter in southern Florida, the West Indies, central Mexico, Central
America, and northwestern South America. In the southern part of
its breeding range it is nesting in April, but those that summer in
New Brunswick do not reach their nesting grounds before the middle
of May. Therefore, about 50 days are required for these northbound
birds to cross the breeding range, and if 60 days be allowed for
nest building, egg laying, incubation, care of young, and molt, they
would not be ready to start southward before the middle of July
(fig. 2.). Then another 50-day trip south, and the earliest migrants
from the northern areas would reach the Gulf Coast in September.
But both adults and young have been observed at Key West, Fla., by
the middle of July, and on the northern coast of South America by
August 21. Since the birds at Key West were fully 500 miles south of
the breeding range, it is evident 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 as the birds nesting in the
northern part of the country take 50 days to cross the breeding
range. The speed of migration is shown in figure 2. (See p. 14.)]
[Illustration: _Figure 2._--Isochronal migration lines of the black
and white warbler, showing a very slow and uniform migration. These
birds apparently advance only about 20 miles a day in crossing the
United States. (See p. 13.)]
Many similar cases might be mentioned, such as the black-throated
blue warblers, which are still observed in the mountains of Haiti in
the middle of May, when others of this species are en route through
North Carolina to breeding territory in New England or have even
reached that region. Redstarts and yellow warblers, evidently the
more southern breeders, are seen returning southward on the northern
coast of South America just about the time that the earliest of
those breeding in the North reach Florida on their way to winter
quarters.
=Nocturnal and diurnal migration=
When one recalls that most birds appear to be more or less helpless
in the dark, it seems remarkable that many should select the night
hours for extended travel. Among those that do, however, are the
great hosts of shore birds, rails, flycatchers, orioles, most of
the great family of sparrows, the warblers, vireos, and thrushes,
and in fact, the majority of small birds. That 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, would
indicate the arrival of migrants during the night. Sportsmen 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 clarion call of the Canada goose, or the conversational
gabbling of a flock of ducks being common night sounds in spring and
fall in many parts of the country. The sibilant, nocturnal calls of
the upland plover or Bartramian sandpiper and of other shore birds
during their spring and fall flights form vivid memories in the minds
of many students of migration. Observations made with telescopes
focused on the full moon have shown processions of birds, one
observer estimating that birds passed his point of observation at the
rate of 9,000 an hour, which gives some indication of the numbers of
birds that are in the air during some of the nights when migration
is at its height. While the steady night-long passage of migratory
birds has been recorded, the bulk of the flocks pass during the
earlier hours of the evening and toward daylight in the morning, the
periods from 8 o'clock to midnight and from 4 to 6 a. m. seeming to
be favorite times for nocturnal flight.
It has been claimed, with some reason, that small birds migrate
by night the better to avoid their enemies, and that most of the
nocturnal travelers are those that are naturally timid, sedentary,
or feeble-winged. To a certain extent this may be true as included
in this group are not only such weak fliers as the rails but also
the small song and insectivorous birds such as the wrens, the small
woodland flycatchers, and other species which, living habitually more
or less in concealment, 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, birds that are always found in the open, and are among
the more powerful fliers, some of them making flights of more than
2,000 miles 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 food be obtained at comparatively short intervals, the
longest of which in most species is during the hours of darkness.
If the smaller migrants were to make protracted flights by day they
would be likely to 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. This would
delay further flights and result in great exhaustion or possibly
even death were they so unfortunate as to have their evening arrival
coincident with unusually cold or stormy weather. Traveling at night,
they pause at daybreak and devote the entire period of daylight to
alternate feeding and resting. This permits complete recuperation and
resumption of the journey at nightfall.
Many species of wading and swimming birds migrate either by day or
night, as they are able to feed at all hours, and are not accustomed
to seek safety in concealment. Some diving birds, including ducks
that submerge when in danger, sometimes travel over water by day and
over land at night. The day migrants include, in addition to some
of the ducks and geese, the loons, cranes, gulls, pelicans, hawks,
swallows, nighthawks, and the swifts. All of these are strong-winged
birds. The swifts, swallows, and nighthawks (sometimes called
bullbats) feed entirely on flying insects, and use their short,
weak feet and legs only for grasping a perch during periods of rest
or sleep. Thus they feed as they travel, the circling flocks being
frequently seen in late summer working gradually southward. Years ago
before birds of prey were so thoughtlessly slaughtered, great flocks
of red-tailed. Swainson's, and rough-legged hawks might be seen
wheeling majestically across the sky in the Plains States, and in the
East the flights of broad-winged. Cooper's, and sharp-skinned hawks
are still occasionally seen. To the birds of prey and possibly to the
gulls also, a day's fasting now and then is no hardship, particularly
since they frequently gorge themselves to repletion when opportunity
is afforded.
[Illustration: _Figure 3._--Migration of the cliff swallow, a day
migrant that instead of flying across the Caribbean Sea as does the
blackpolled warbler (see fig. 6), follows around the coast of Central
America, where food is readily obtained. (See p. 25.)]
An interesting comparison of the flights of day and night migrants
may be made through a consideration of the spring migrations of the
blackpolled warbler and the cliff swallow. Both spend the winter
in South America, at which season they are neighbors. But when the
impulse comes to start northward toward their respective breeding
grounds, the warblers strike straight across the Caribbean Sea
to Florida (fig. 6), while the swallows begin their journey by a
westward flight of several hundred miles to Panama (fig. 3.). Thence
they move leisurely along the western shore of the Caribbean Sea
to Mexico, and continuing to avoid a long trip over water, they
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 blackpolled warbler?" The simple explanation
is that 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 a few hours of aerial evolutions, which at the
same time carry it slowly in the proper direction. Flying along the
insect-teeming shores of the Gulf of Mexico, the 2,000 extra miles
that are added to the migration route are but a fraction of the
distance that these birds cover in pursuit of their daily food.
Although most of our smaller birds make their longest flights at
night, close observation will show that travel is continued to some
extent by day. This is particularly true during the latter half of
a migratory season when the birds show evidence of an overpowering
desire to hasten to their breeding grounds. At this time flocks of
birds while feeding maintain a movement in the general direction of
the seasonal journey. Sometimes they travel hurriedly, and while
their flights may be short, they must cover an appreciable distance
in the course of a day.
_How Birds Migrate_
=Speed of flight and speed of migration=
There is a widespread misconception concerning the speed at which
birds normally fly, and even regarding the speed they can attain when
occasion demands, as when closely pursued by an enemy. It is not
unusual to hear accounts of birds flying "a mile a minute." While
undoubtedly some birds can and do attain a speed even greater than
this, such cases are exceptional, and it is safe to say that even
when pressed, few can develop an air speed of 60 miles an hour. They
do, however, have two speeds, one being the normal rate for everyday
purposes and also for migration, and an accelerated speed for escape
or pursuit; this in some cases may be nearly double the normal rate
of movement. Nevertheless, it is doubtful if the effort required for
the high speeds could be long sustained, and certainly not for the
long-distance migratory journeys that are regularly made by most
birds. The theory that migrating birds attain high speeds received
encouragement from the German ornithologist Gätke (1895), who for
many years made observations on birds at the island of Heligoland.
He postulated that the blue-throat, a species of thrush smaller than
the American hermit thrush, would leave African winter quarters at
dusk and reach Heligoland at dawn, which would mean a sustained speed
of 200 miles an hour, and that the American golden plover flew from
the coast of Labrador to Brazil in 15 hours, or at the tremendous
speed of 250 miles an hour. Most ornithologists now consider these
conclusions to be unwarranted.
Sportsmen also often greatly overestimate the speed at which ducks
and geese fly and sometimes attempt to substantiate their estimates
by mathematical calculation, based upon the known velocity of a
charge of shot, the estimated distance and the estimated "lead" that
was necessary to hit the bird. If all three elements of the equation
were known with certainty, the speed of the bird could be determined
with a fair degree of accuracy. The majority of the ducks that are
reported as killed at 40, 50, or even 60 yards, however, actually
are shot at distances much less than estimated. To sight along a gun
barrel and estimate correctly the distance of a moving object against
the sky is so nearly impossible for the average gunner as to make
such calculations of little value.
During the past few years reliable data on the speed of birds have
accumulated slowly. It has been found that a common flying speed
of ducks and geese is between 40 and 50 miles an hour, and that it
is much less among the smaller birds. Herons, hawks, horned larks,
ravens, and shrikes, timed with the speedometer of an automobile,
have been found to fly 22 to 28 miles an hour, while some of the
flycatchers are such slow fliers that they attain only 10 to 17 miles
an hour. Even such fast-flying birds as the mourning dove rarely
exceed 35 miles an hour. All these birds can fly faster, but it is to
be remembered that at training camps during World War I, airplanes
having a maximum speed of about 80 miles an hour easily overtook
flocks of ducks that, it may be supposed, were making every effort
to escape. Aviators have claimed that at 65 miles an hour they can
overtake the fastest ducks, though cases are on record of ducks
passing airplanes that were making 55 miles an hour.
The greatest bird speeds that have been reliably recorded are of the
swifts and the duck hawk, or peregrine falcon. An observer in an
airplane in Mesopotamia reported that swifts easily circled his ship
when it was traveling at 68 miles an hour. To do this, the birds
certainly were flying at a speed as high as 100 miles an hour. Once
a hunting duck hawk, timed with a stop watch, was calculated to have
attained a speed between 165 and 180 miles an hour.
The speed of migration, however, is quite different from that
attained in forced flights for short distances. A sustained flight of
10 hours a 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. Measured as air-line distances,
these journeys are impressive and indicate that birds could cover the
ordinary migration route from the northern United States or even from
northern Canada to winter quarters in the West Indies or in Central
America 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. Nevertheless, whether they continue such journeys day
after day is doubtful.
It seems more likely that migrations are performed in a leisurely
manner, and that after a flight of a few hours the birds 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 records of banded birds, particularly waterfowl.
Considering only the shortest intervals that have elapsed between
banding in the North and recovery in southern regions, it is found
that usually a month or more is taken to cover an air-line distance
of a thousand miles. For example, a black duck banded at Lake Scugog,
Ontario, was killed 12 days later at Vicksburg, Miss. If the bird was
taken shortly after its arrival, the record would indicate an average
daily flight of only 83 miles, a distance that could have been
covered in about 2 hours' flying time. Among the thousands of banding
records obtained in recent years, evidences of such rapid flight 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 which traveled 3,800 miles from the delta of the Athabaska
River, in northern Alberta, Canada, to Maracaibo, Venezuela, in
exactly 1 month. This flight was at an average speed of 125 miles per
day. The greatest migration speed thus far recorded for any banded
bird is that of a lesser yellowlegs banded at North Eastham, Cape
Cod, Mass., on August 28, 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 certain that migratory journeys are performed at the normal
rate of flight, as this would best conserve the strength of the birds
and eliminate the fatigue that would result from effort required for
great speed. 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 of migration also 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
at this season is at the same rate as the advance of the season (fig.
4). In fact, the isotherm of 35° F. appears to be a governing factor
in the speed at which these geese move north, and over their entire
trip the vanguard follows closely the advance of this isotherm.
Few species perform such regular migrations, many waiting in their
winter homes until spring is well advanced and then moving rapidly
to their breeding grounds. Sometimes this advance is so rapid that
the later migrants actually catch up with species that for a month or
more may have been pressing slowly but steadily northward.