Many species annually migrate to take advantage of global differences of seasonal temperatures, to find availability of food sources and breeding habitat. Shorter migrations are for avoiding bad weather and to obtain food.
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 summer months at locations all throughout the United States and Canada. Bird experts propose the following reasons for the birds' return north to breed each spring:
- In the north, there is an abundant supply of insects which birds can feed their young.
- The number of hours of daylight increases at points farther north, during summer, in the Northern Hemisphere.
- There is less competition for food and nesting sites in the north.
- In the north, there are fewer mammals that will prey on nesting birds.
- During the summer months, weather conditions in the north are comfortable while conditions in the south grow unbearably hot.
Sources: Feldman, David. Do Penguins Have Knees? pp. 147-48; Terres, John K. The Audubon Society Encyclopedia of North American Birds, pp. 602-5; World Book Encyclopedia, vol. 2, pp. 360-61.