Читать книгу Migration of Birds (1979) онлайн

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Modern views based on studies of bird behavior and physiology indicate migration is a regular, annually induced movement, modified by local weather conditions, but largely independent of them. Migration is a phenomenon far too regular to be created anew each season merely under stress of circumstances, such as need for food; and it begins before the necessity for a change in latitude becomes at all pressing. Swallows, nighthawks, shorebirds, and others may start their southward movement while the summer food supply in the North is at peak abundance. American robins and bluebirds may leave abundant food in the South and press northward when food supplies there are almost entirely lacking and severe cold and storms are likely to cause their wholesale destruction. Regularity of arrival and departure is one of the most impressive features of migration, and since birds travel in a rather strict accordance with the calendar, we might ask: "What phenomena, other than the regular changes in length of day, occur with sufficient precision to act as a stimulus for migration?"