Food, water, protective cover, and a sheltered place to nest and breed are basic to a bird's survival. But the changing seasons can transform a comfortable environment into an unlivable one -- the food and water supply can dwindle or disappear, plant cover can vanish, and competition with other animals can increase.

Most wild animals face the problem of occupying a habitat that is suitable for only a portion of the year. Fortunately, however, nature has provided methods for coping with the situation. One method, known as hibernation, involves entering a dormant state during the winter season. The other method, known as migration, involves escaping the area entirely. Because of the powers of flight, most birds adapt to seasonal changes in the environment by migrating; only a few birds species, such as the Common Poorwill, hibernate.

The annual fall migration of the Townsend's Solitaire may consist merely of descending a few thousand feet from a high mountain forest to the shelter of a wooded valley.