Science Fact Finder | The Animal World - Why Do Birds Migrate Annually?

Why do birds migrate annually?

Migratory behavior in birds is inherited; however, birds will not migrate in the absence of certain physiological and environmental cues. In the late summer, the decrease in sunlight stimulates a migrating bird's pituitary gland to produce the hormone prolactin and its adrenal gland to produce the hormone corticosterone. These hormones, in turn, cause the birds to accumulate large amounts of fat just under the skin, providing them with enough energy for the long migratory flights. The hormones also cause the birds to become restless just prior to migration. The exact time of departure, however, is dictated not only by the decreasing sunlight and hormonal changes, but also by such conditions as the availability of food and the onset of cold weather.

The major wintering areas for North American migrating birds are the southern United States and Central America. These birds spend the...

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