Читать книгу The Migration of North American Birds (1935) - Frederick Charles Lincoln - Страница 13

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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.