Читать книгу Migration of Birds (1979) - Frederick Charles Lincoln - Страница 17

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Some species begin their fall migrations early in July, and in other species distinct southward movements can be detected late into the winter. While some migrants are still traveling south, some early spring migrants can be observed returning north through the same locality. For example, many shorebirds start south in the early part of July, while the goshawks, snowy owls, redpolls, and Bohemian waxwings do not leave the North until forced to do so by the advent of severe winter weather or a lack of 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 migrants, the northern movement may continue well into 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 and may actually start south while others of the species are still on their way north.

A study of these facts indicates the existence of northern and southern populations of the same species that have quite different migration schedules. In fall, migratory populations that nest farthest south migrate first to the winter range because they finish nesting first. For example, the breeding range of the black-and-white warbler covers much of the eastern United States and southern Canada northwest through the prairies to Great Bear Lake in Canada (Fig. 1.). It spends the winter in southern Florida, the West Indies, southern and eastern Mexico, Central America, and northwestern South America. In the southern part of its breeding range, it nests in April, but those summering in New Brunswick do not reach their nesting grounds before the middle of May. (Lines that connect points where birds arrive at the same line are called isochronal lines. Fig. 2.) Therefore, if 50 days are required to cross the breeding range, and if 60 days are allowed for reproductive activities and molting, they would not be ready to start southward before the middle of July. Then with a return 50-day trip south, the earliest migrants from the northern areas would reach the Gulf Coast in September. Since adults and young have been observed on the northern coast of South America by August 21, it is very likely 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, these birds nesting in the northern part of the country take 50 days to cross the breeding range. The speed of migration is shown in Fig. 2.

Many similar cases might be mentioned, such as the black-throated blue warblers still observed in the mountains of Haiti during the middle of May when others of this species are en route through North Carolina to New England breeding grounds. Redstarts and yellow warblers, evidently the more southern breeders, are seen returning southward on the northern coast of South America just about the time the earliest of those breeding in the North reach Florida on their way to winter quarters. Examples of the Alaska race of the yellow warbler have been collected in Mississippi, Florida, and the District of Columbia as late as October.

Figure 2. Isochronal migration lines of the black-and-white warbler, showing a very slow and uniform migration. The solid lines connect places at which these birds arrive at the same time. These birds apparently advance only about 20 miles per day in crossing the United States.

Students of migration know that birds generally travel in waves, the magnitude of which varies with populations, species, weather, and time of year. Characteristically, one will observe a few early individuals come into an area followed by a much larger volume of migrants. This peak will then gradually taper off to a few lingering stragglers. If we plot numbers observed against time, the rising and receding curve takes the form of a bell. In the northern part of the United States there are two general migration waves. The first one in early spring consists of "hardy" birds including many of our common seed eaters like the finches, sparrows, and others. The second wave occurs about a month later and consists primarily of insect-eating birds, such as flycatchers, vireos, warblers, and the like. Each of these species in turn has its own "curve" of migration in the major wave.