correlated with the periodic maximum of Arctic foxes and the lemming
cycle in the north. 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 as 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, at which
time the writer 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 performing more or less
wandering journeys, and curiously enough, in addition to occasional
trips south of their regular range, they travel east and west,
sometimes covering long distances. For example, grosbeaks banded at
Sault Ste. Marie, Mich., have been recaptured on Cape Cod, Mass., and
in the following season have been retrapped at the banding station.
Banding records demonstrate that this east-and-west trip across the
northeastern part of the country is sometimes made also by purple
finches.
_Perils of Migration_
The period of migration is a season full of peril for birds. Untold
thousands of the smaller migrants are destroyed each year by storms
and through attacks of predatory birds, mammals, and reptiles. If
each pair of adult birds should succeed in raising two fledglings to
maturity, the population of migratory birds would have a potential
annual increase of 100 percent and the world would soon be heavily
overpopulated with them. Since there is no such increase it is
evident that the annual mortality from natural causes is heavy enough
to keep it in check.
=Storms=
Of the various factors limiting the abundance of birds, particularly
the smaller species, storms are the most potent. Special sufferers
are those birds that in crossing broad stretches of water are forced
down by a storm within reach of 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, had accomplished nearly 95 percent of
their long flight and were nearing land, when, caught by a norther
against which they were unable to contend, hundreds were forced into
the waters of the Gulf and drowned. On another occasion, on Lake
Michigan, a severe storm came up at a time when large numbers of
migratory birds were crossing and forced numerous victims into the
waves. During the fall migration of 1906, when thousands of birds
were crossing Lake Huron, a sudden drop in temperature accompanied
by a heavy snowfall resulted in the death of incredible numbers.
Literally thousands were forced into the water and subsequently cast
up along the beaches, where in places their bodies were piled in
windrows. On one section of the beach the dead birds were estimated
at 1,000 per mile, and at another point at 5 times that number. Most
of them were species that rank among our most desirable birds as
destroyers of insects and weed seeds, including slate-colored juncos,
tree sparrows, white-throated sparrows, swamp sparrows, winter wrens,
and golden-crowned kinglets, together with many brown creepers,
hermit thrushes, warblers, vireos, and others.
Of all species of North American birds, the Lapland longspur seems to
be the most frequent victim of mass destruction from storms. These
birds sometimes congregate in enormous numbers where grass or weed
seed is abundant. Almost every winter brings in reports of their
death by thousands somewhere in the Middle West. While migrating
northward at night they have encountered blinding storms of wet,
clinging snow, which have so bewildered them that they have flown
into various obstructions, or have sunk to the ground and perished
of exposure and exhaustion. In 1907 an experienced ornithologist
estimated that 750,000 longspurs were lying dead on the ice of 2
lakes in Minnesota, each about 1 square mile in extent, and dead
birds were reported in greater or less abundance on this occasion
over an area of more than 1,500 square miles. The heaviest mortality
occurred in towns, where, bewildered by the darkness and the heavy
falling snow, some of the birds congregating in great numbers flew
against various obstacles and were killed or stunned, while many
others fell to the ground exhausted. Similar catastrophes have been
reported from eastern Colorado, Nebraska, and North Dakota.
During the early part of June 1927, a hailstorm of exceptional
severity in and around Denver, Colo., killed large numbers of robins,
meadow larks, sparrows, and others. The lawns of parks were strewn
with the bodies of these birds, and many lay dead in their nests
where they were covering their eggs or young when the storm broke.
=Aerial obstructions=
Lighthouses, lightships, tall bridges, piers, monuments, and
other aerial obstructions have been responsible for a tremendous
destruction of migratory birds. Beams of the lanterns at light
stations have a powerful attraction for nocturnal travelers of the
air that may be likened to the fascination for lights that also
is shown by many insects, particularly night-flying moths. The
attraction is not so potent in clear weather, but when the atmosphere
is moisture laden, as in a heavy fog, the rays have a dazzling effect
that lures the birds to their death. They may fly straight up the
beam and dash themselves headlong against the glass, or they may keep
fluttering around the source of the light until exhausted, and then
drop to the rocks or waves below. The fixed, white, stationary light
located 180 feet above sea level at Ponce de Leon Inlet (formerly
Mosquito Inlet), Fla., has caused great destruction of bird life
even though the lens is shielded by wire netting. On one occasion an
observer gathered up a bushel-basketful of warblers, sparrows, and
other small passerine birds that had struck during the night. The
birds apparently beat themselves to death against the wire or fell
exhausted to the concrete pavement below, frequently to be destroyed
there by cats or skunks. 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. It is the fixed white lights that cause such disasters
to birds, as the stations equipped with flashing or red lights do
not present such strong attractions. That it is not a mere case of
geographical location has been demonstrated, for it is observed that
when fixed white lights have been changed to red or flashing lights,
the migrating birds are no longer endangered. At some of the light
stations in England and elsewhere, shelves and perches have been
placed below the lanterns to afford places where birds can rest until
they have overcome their bewilderment.
For many years at the National Capital, the Washington Monument,
although unilluminated, caused the destruction of large numbers of
small birds, due apparently to their inability to see this obstacle
in their path, towering more than 555 feet into the air. One morning
in the spring of 1902 the bodies of nearly 150 warblers, sparrows,
and other birds were found about its base. Then, as the illumination
of the city was improved and the Monument became more visible at
night, the loss became steadily less, until by 1920 only a few birds
would be killed during an entire migration. On November 11, 1931,
however, as part of the Armistice Day celebration, batteries of
brilliant floodlights grouped on all four sides about the base of the
Monument were added to the two searchlights already trained on the
apex, so that the lighted shaft probably corresponds in brilliancy
to a very low magnitude lighthouse lantern. Airplane pilots have
ventured opinions that on a clear night it could be seen for 40
miles. It is certain that there is an extensive area of illumination,
and on dark nights, when there are gusty, northerly winds and the
nocturnal travelers seem to fly at lower altitudes, many of them are
attracted to the Monument as to a lighthouse beacon. As they approach
from the north a last-minute attempt to avoid it causes them to
veer off to the east or the west where they are literally sucked in
and dashed against the southern face of the shaft. During the fall
migration of 1932 more than 500 warblers, vireos, thrushes, kinglets,
sparrows, and others were killed. Since that year the mortality has
been less, but the Monument at times remains a serious menace to
birds during migration and some are killed nearly every fall.
When the torch on the Statue of Liberty in New York Harbor was kept
lighted, it caused an enormous destruction of bird life, tabulations
showing as many as 700 birds killed in a single month.
In September 1948, bird students were startled by news of the
wholesale destruction of Maryland yellowthroats, redstarts,
ovenbirds, and others that were dashed 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 over a 6-hour
period and their bodies were scattered over a four-block area. The
mortality was so heavy in Philadelphia that it was impossible to
use the sidewalk below the sky-scraper until the birds had been
gathered. A study of the weather conditions prevailing at this
time in the Atlantic coastal region suggests the probable cause
of this catastrophe. By early morning on September 11 a mass of
cold, southward-flowing air had just reached New York City where
it was forcing upward and was being overridden by a mass of warm,
northward-flowing air. Presumably the migrants were riding the upper
levels of the southbound current which, in the contact zone with the
northbound current, was being deflected earthward, thus causing the
birds to fly lower and lower until they were below the tower of the
Empire State Building. Clouds and gusty winds in the zone of contact
between the two air masses reduced visibility and disrupted avian
navigation with the result that the confused travelers crashed into
the stone and steel obstruction. As the cold air mass continued to
move southward, the situation was repeated at Philadelphia and at
Baltimore.
=Exhaustion=
Although it would seem that the exertion incident to the long
flights of many species of migratory birds would result in their
arrival at their destination in a state bordering on exhaustion,
this is contrary to the truth. Both the soaring and the sailing of
birds show them to be proficient in the use of factors employed in
aerial transportation that only recently have become understood and
imitated by aeronautical engineers. The use of ascending currents
of air, employed by all soaring birds, and easily demonstrated by
observing the gulls that 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 renders it the most perfect
machine for extensive flight that the world has ever known. Hollow,
air-filled bones, making an ideal combination of strength and
lightness, and the lightest and toughest material possible for
flight in the form of feathers, combine to produce a perfect flying
machine. Mere consideration of a bird's economy of fuel or energy
also is enlightening. The golden plover, traveling over the oceanic
route, makes the entire distance of 2,400 miles from Nova Scotia to
South America without stop, probably requiring about 48 hours of
continuous flight. This is accomplished with the consumption of less
than 2 ounces of fuel in the form of body fat. To be as economical in
operation, a 1,000-pound airplane would consume in a 20-mile flight
not the gallon of fuel usually required, but only a single pint.
The sora, or Carolina rail, which is such a notoriously weak flyer
that at least one writer was led to infer that most of its migration
was made on foot, has one of the longest migration routes of any
member of the family, and easily crosses the wide reaches of the
Caribbean Sea. The tiny ruby-throated hummingbird crosses the Gulf of
Mexico in a single flight of more than 500 miles.
While birds that have recently arrived from a protracted flight
over land or sea sometimes show evidences of being tired--as, for
example, pintail ducks that have flown from the North American
mainland to the Hawaiian Islands--their condition is far from being
a state of exhaustion. With a few hours' rest and a crop well
filled with proper food, most birds exhibit eagerness to resume
their journey. The popular notion that birds find the long ocean
flights excessively wearisome and that they sink exhausted when
terra firma is reached, generally does not agree with the facts.
The truth lies in the opposite direction, as even small land birds
are so little averse to ocean voyages that they not only cross the
Gulf of Mexico at its widest point, but may even pass without pause
over the low, swampy coastal plain to the higher regions beyond.
Under favorable conditions birds can fly when, where, and how they
please. Consequently the distance covered in a single flight is
governed chiefly by the food supply. Exhaustion, except as the result
of unusual factors, cannot be said to be an important peril of
migration.
_Influence of the Weather on Migration_
The state of the weather at any point has little if anything to do
with the time of arrival of migratory birds. This is contrary to the
belief of observers who have thought that they could foretell the
appearance of various species by a study of the weather conditions.
Though the insistent crescendo note of the ovenbird is ordinarily
associated with the full verdure of May woods, 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 degrees between highest and
lowest temperatures to 37 degrees, the average variation being about
24 degrees.
It should be remembered that North American species spending the
winter months in tropical latitudes experience no marked changes in
climatic 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 that the urge to migrate
is so ingrained that each species moves north in spring when the
average weather that will be encountered is not unendurable. The
word "average" must be emphasized since it appears obvious that the
migrations of birds have so evolved that in general they synchronize
with average climatic conditions. 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,
and rarely in the destruction of large numbers of individuals. Cases
are known where early migrating bluebirds have been overwhelmed by
late winter storms. Nevertheless, unless such climatic conditions are
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.
As has been pointed out, the advance of average temperature lines,
known as isotherms, is found to correspond closely with the northward
movements of certain species. For example, the northward travels
of the Canada goose are found to coincide with the advance of the
isotherm of 35° F. (fig. 4).
The spring flight of migrants, if interrupted for any reason, is
resumed when weather conditions again become favorable, and it is
probable that all instances of arrival of birds in stormy weather
can be explained on the theory that the flight was begun while the
weather was auspicious. The state of the weather when a flight starts
at any southern point, the relation of that place to the average
position of the bird under normal weather conditions on that date,
and the average rate of migratory flight, are data basic to any
reasonably accurate prediction of the time arrival may be expected in
northern areas.
Head winds are as unfavorable to migration as is rain or snow, as
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 areas, large numbers of birds are lost. Even strong winds that
blow in the direction of aerial travel are unfavorable for the birds,
as they interfere with their balance and disarrange their feathers.
Moderate tail winds and cross or quartering breezes appear to offer
the best conditions for the passage of the migrants.
_Problems of Migration_
=Banding studies=
The study of living birds by the banding method, whereby great
numbers of individuals are marked with numbered aluminum leg rings,
has come to be recognized as a most accurate means of ornithological
research. Since 1920, banding work in North America has been under
the direction of the Fish and Wildlife Service in cooperation
with the Dominion Wildlife Service of Canada. Every year voluntary
cooperators, working under permit, place bands on thousands of birds,
game and nongame, large and small, migratory and nonmigratory,
each band carrying a serial number and the legend, NOTIFY FISH AND
WILDLIFE SERVICE, WASHINGTON, D. C., or on the smaller sizes an
abbreviation thereof. When a banded bird is reported from a second
locality, a definite fact relative to its movements becomes known,
and a study of many cases of this nature 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 the exact dates
of arrival and departure of individuals, the length of time that
different birds pause on their migratory journeys to feed and rest,
the relation between weather conditions and the starting times for
migration, the rates of travel of individual birds, the degree of
regularity with which birds return to the exact summer or winter
quarters used in former years, and many other details that could
be learned in no other manner. Banding stations that are operated
systematically throughout the year, therefore, are supplying much
information concerning the movements of migratory birds that
heretofore could only be surmised. (See Appendix II, p. 92 for
instructions on reporting the recovery of banded birds.)
=Movements of residents=
Typical migration consists of definite movements that are repeated
regularly year after year, and it is to these that the term is
generally restricted. It is desirable, however, if only for purposes
of comparison, that some account be taken of the movements of some
other birds, which, while not typical, do possess some of the
characteristics of true migration. Data on this subject are being
collected through bird banding.
There are several species that are customarily grouped under the
heading "permanent residents," the term implying that these birds
do not travel but remain throughout the year in one locality.
Among these are the cardinal, the tufted titmouse, the wrentit,
the Carolina wren, the house finch, the bobwhite, the California
quail, and the ruffed grouse. Each species may be present constantly
throughout the year, although in the northern part of the range there
is probably a slight withdrawal of the breeding birds in winter. The
individuals to be seen at that season, therefore, may not always be
the same as those observed during the summer. It is certain, however,
that these species do not regularly perform extensive journeys.
While the blue jay is disposed to be secretive, it is such a showy
and noisy bird that it is not likely to escape notice. In the
vicinity of Washington, D. C., as in many other places, it is present
the year round, but at the end of September or early in October
when the weather is becoming cooler, troops of jays are sometimes
seen working southward through the trees. A corresponding northward
movement occurs again in May. This is unquestionably a migration to
and from some winter range, but its extent or significance is not now
known. Some light is being shed on the matter, however, through the
records of banded birds, and these eventually will fill in a more
perfect picture of the movements of this species. One jay, banded on
September 14, 1923, at Waukegan, Ill., was killed at Peruque, Mo.,
on November 15 of the same year; another, banded at Winnetka, Ill.,
on June 16, 1925, was retaken at Sulphur Rock, Ark., the following
December 10; a third, banded on May 6, 1925, at Whitten, Iowa, was
recaptured at Decatur, Ark., on January 22, 1926. These three birds
unquestionably had made a flight that had every appearance of being a
true migration to winter quarters in Missouri and Arkansas.
The black-capped chickadee is apparently resident in many places, but
occasionally in winter it invades the range of the southern Carolina
chickadee, and in northern Canada it is regularly a migrant.
In the coastal plain between Washington, D. C., and the Atlantic
Ocean, the white-breasted nuthatch is usually absent during the
summer, nesting at that season in the higher, or piedmont, country.
Late in fall, however, it appears in fair abundance in the wooded
bottoms, remaining at the lower levels until the following March or
April.
Some birds, including the screech owl, bobwhite, Carolina wren,
and mockingbird, seem to be actually sedentary, but even these are
sometimes given to post-breeding wanderings. Ordinarily bobwhites
that are marked with numbered bands are seldom retaken far from the
area where banded, but sometimes they will travel 10 miles or more.
A screech owl banded at Glenwood, Minn., in March, was recovered
the following December at Emmetsburg, Iowa, 180 miles south. Such
flights, however, are probably more in the nature of a search for
new feeding areas, or to escape from a winged enemy, than a true
migratory journey.
=Migration of the white-throated sparrow=