Читать книгу Migration of Birds (1979) онлайн

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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.

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Because most birds appear to be creatures of daylight, it seems remarkable that many should select the night for extended travel. Among the many nocturnal migrants are the smaller birds such as rails, flycatchers, orioles, most of the sparrows, the warblers, vireos, thrushes, and shorebirds. 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, which indicates the arrival of migrants during the night. Waterfowl hunters 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 calls of Canada geese or the conversational gabbling of a flock of ducks are common night sounds in spring and fall in many parts of the country. Observations made with telescopes focused on the full moon have shown processions of birds, and one observer estimated their passage over his area at the rate of 9,000 per hour. This gives some indication of the numbers of birds in the air at night during peaks of migration. At such times radar observations have shown that nocturnal migration begins about an hour after sundown, reaches a peak shortly before midnight, and then gradually tapers off until daybreak. Unless special circuits are installed in radar sets, bird echoes during peak migration periods may cover a radar screen.