Читать книгу Migration of Birds - Frederick Charles Lincoln - Страница 7
ОглавлениеWhen Birds Migrate
It is known that at any given point many species leave in the fall and return in the spring. Since banding has had such wide application as a method of study, it is known also that in some species one of the parent birds (rarely both) frequently returns and nests in the tree, bush, or box that held its nest in the previous season. One ordinarily thinks of the world of birds as quiescent during two periods each year, at nesting time, and in winter. For individuals this is obviously the case, but when the entire avifauna of the continent is considered it is found that there are at almost all periods some latitudinal movements.
Movements of species and groups
Some species begin their fall migrations early in July and in some parts of the country distinct southward movements can be detected from then until the beginning or middle of winter. For example, many shore birds start south in the early part of July, while the goshawks, snowy owls, redpolls, Bohemian waxwings, and many others do not leave the North until forced to do so by the advent of severe winter weather, or by lack of the customary food. Thus an observer in the northern part of the United States may record an almost unbroken southward procession of birds from midsummer to winter, and note some of the returning migrants as early as the middle of February. While on their way north, purple martins have been known to arrive in Florida late in January and, among late arrivals, the northern movement may continue into the first week of June. In some species the migration is so prolonged that the first arrivals in the southern part of the breeding range will have performed their parental duties while others of that species are still on their way north.
A study of these facts indicates that sometimes there exists a very definite relationship between what we may term northern and southern groups of individuals of the same species. A supposition is that for a species with an extensive latitudinal breeding range, and which has a normal migration, those individuals that nest farthest south migrate first and proceed to the southern part of the winter range; those that occupy the central parts of the breeding range migrate next and travel to regions in the winter range north of those occupied by the first group; and finally the individuals breeding farthest north are the last to start their autumn migration and they remain farthest north during the winter. In other words, this theory supposes that the southward movement of the species is such that the different groups maintain their relative latitudinal position with each other. The black and white warbler furnishes an example. The breeding range of this bird extends west and northwest from northern Georgia and South Carolina to New Brunswick, extending also in a western and northwestern direction as far as Great Bear Lake in northwestern Canada (fig. 1). It spends the winter in southern Florida, the West Indies, central Mexico, Central America, and northwestern South America. In the southern part of its breeding range it is nesting in April, but those that summer in New Brunswick do not reach their nesting grounds before the middle of May. Therefore, about 50 days are required for these northbound birds to cross the breeding range, and if 60 days be allowed for nest building, egg laying, incubation, care of young, and molt, they would not be ready to start southward before the middle of July (fig. 2.). Then another 50-day trip south, and the earliest migrants from the northern areas would reach the Gulf Coast in September. But both adults and young have been observed at Key West, Fla., by the middle of July, and on the northern coast of South America by August 21. Since the birds at Key West were fully 500 miles south of the breeding range, it is evident that they must have come from the southern part of the nesting area.
Figure 1.—Summer and winter homes of the black and white warbler, a very slow migrant as the birds nesting in the northern part of the country take 50 days to cross the breeding range. The speed of migration is shown in figure 2. (See p. 14.)
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.)
An interesting comparison of the flights of day and night migrants may be made through a consideration of the spring migrations of the blackpolled warbler and the cliff swallow. 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 (fig. 6), while the swallows begin their journey by a westward flight of several hundred miles to Panama (fig. 3.). 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 blackpolled 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 daily food.
Although most of our smaller birds make their longest flights at night, close observation will show that travel is continued to some extent by day. This is particularly true during the latter half of a migratory season when the birds show evidence of an overpowering 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 must cover an appreciable distance in the course of a day.
