Читать книгу Migration of Birds (1979) онлайн

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One ordinarily thinks of the world of birds as sedentary during two periods each year, at nesting time, and in winter. For individuals this is obviously the case, but when the entire avifauna of North America or the world is considered, it is found that at almost all periods there are some latitudinal movements of birds. A few of these movements reoccur year after year with calendar-like regularity. Each species, or group of species, migrates at a particular time of the year and some at a particular time of the day. In this section some of the interesting differences will be discussed as to when birds migrate.

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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.