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 (fig. 4), 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.
The passage of flocks of ducks and geese is frequently observed by
sportsmen sitting in their blinds, but great numbers of these birds
also pass through at night, the clarion call of the Canada goose
(_Branta canadensis_) 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 (_Bartramia longicauda_), and 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. The
estimate of one observer that birds passed his point of observation
at the rate of 9,000 an hour 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 passage of migratory birds has thus been
recorded throughout the night, 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. Included in this group are not only small song and
insectivorous birds, but also such weak fliers as the rails, as well
as the wrens, the small woodland flycatchers, and other species,
which, living habitually more or less in concealment, are probably
much safer making their long flights under the protecting cloak of
darkness. This cannot fully account for the nocturnal habit, however,
since among the night migrants are the snipe, sandpipers, and
plovers, birds that are generally 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. Such exceptionally long flights, of course,
require both day and night flying.
Night travel is probably best for the majority of birds, chiefly
from the standpoint of feeding. Digestion is rapid in birds, and
yet the stomach of a bird killed during the day almost always
contains food. To supply 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 unable to obtain food until the following
morning, since they are entirely daylight feeders. This would delay
resumption of flight 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 indifferently
by day or by 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 1 and at night. The day migrants include,
in addition to some of the ducks and geese, the loons, cranes,
gulls, pelicans, hawks (fig. 7), swallows, nighthawks, and the
swifts (fig. 6), all 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 late in summer
working gradually southward. Years ago, before birds of prey were so
thoughtlessly slaughtered, great flocks of red-tailed hawks (_Buteo
borealis_), Swainson's hawks (_B. swainsoni_), and rough-legged
hawks (_B. lagopus_ and _B. regalis_) might be seen wheeling
majestically across the sky in the Plains States, and in the East
the flights of broad-winged hawks (_B. platypterus_), Cooper's hawks
(_Accipiter cooperi_), and sharp-shinned hawks (_A. velox_) are
still occasionally seen, although these birds do not actually travel
in flocks. 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.
The spring migrations of the blackpoll warbler (_Dendroica striata_)
and the cliff swallow (_Petrochelidon albifrons_) afford an
interesting comparison of the flights of day and night migrants.
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, while the swallows begin their journey
by a westward flight of several hundred miles to Panama (fig. 5).
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 blackpoll 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 food.
Although most of our smaller birds make their longest flights at
night, close observation shows that travel is continued to some
extent by day. This is particularly the case during the latter half
of a migratory season, when the birds manifest a 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 cover considerable distances in the course of a day.
=HOW BIRDS MIGRATE=