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 (_Porzana Carolina_), 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 its family, and easily crosses the
wide reaches of the Caribbean Sea. The tiny ruby-throated hummingbird
(_Archilochus colubris_) 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, their condition
is far from a state of exhaustion, unless unusual conditions have
been encountered, and with a few hours' rest and a crop well filled
with proper food they 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
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
4 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 recorded at the
time of arrival of several other common birds show variations of 14°
to 37° F., the average variation being about 24°.
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.
Through their influence upon the food supply, weather conditions in
the breeding areas do, of course, figure prominently in the picture.
In fact, they are the major factors in determining the average
dates of arrival of the different species. The word "average" must
be emphasized, for the migrations of birds have so evolved that in
general they synchronize with average climatic conditions. In other
words, migration is so ingrained that each species moves north in
spring when the average weather that will be encountered is not
unendurable. The hardy birds travel early, fearless of the blasts
of retreating winter. The more delicate species come later, when
there is less danger of encountering prolonged periods of inclement
weather. Some of these hardy birds pause and allow the spring season
to advance, and then by rapid travel again overtake it, or in some
cases actually outstrip it. At times this results in some hardship
and occasionally in the destruction of large numbers of individuals,
as has happened when early migrating bluebirds have been overwhelmed
by a late winter storm. Unless such conditions are prolonged,
however, no serious effect on the species is noted. Nevertheless,
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 population.
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. 8).
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 should this happen over
water areas, large numbers of birds would be 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.
[Illustration: B3712M
Figure 28.--A banded belted kingfisher, one of about 2,000,000 birds
that have been marked with Biological Survey bands.]
=PROBLEMS OF MIGRATION=