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.
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.])