Migratory behavior in birds is inherited; however, birds will not migrate without certain physiological and environmental stimuli. In the late summer, the decrease in sunlight stimulates the pituitary gland and the adrenal gland of migrating birds, causing them to produce the hormones prolactin and corticosterone respectively. 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. Migrating ducks follow four major flyways south: the Atlantic flyway, the Mississippi flyway, the central flyway, and the Pacific flyway. Some bird experts propose that the birds return north to breed for several reasons: (1) Birds return to nest because there is a huge insect supply for their young; (2) The higher Earth’s latitude in the summer in the Northern Hemisphere, the longer the daylight available to the parents to find food for their young; (3) Less competition exists for food and nesting sites in the north; (4) In the north, there are fewer mammal predators for nesting birds (which are particularly vulnerable during the nesting stage); (5) Birds migrate south to escape the cold weather, and they return north when the weather is more temperate.