Climate also influences a bird's range. Most birds would starve during a long cold spell. For this reason, few birds live all year in regions with severe winters. However, many birds nest in such regions in summer and migrate to warmer climates for the winter. Birds that migrate have two ranges-a summer one and a winter one. They are summer residents in their summer range and winter residents in their winter one. Along their migration route, they are transients (temporary visitors). Birds that do not migrate are permanent residents.
More kinds of birds live in the tropics than anywhere else in the world. Tropical rain forests have more kinds of birds than any other habitat. Most birds of the tropics are permanent residents. However, some parts of the tropics have an annual dry season, and many of the birds migrate to moister parts of the tropics to avoid it. The tropics also have many winter residents that migrate from cool or cold climates. The temperate zones-that is, the parts of the world between the tropics and the polar regions-have fewer permanent residents than do the tropics. In the parts of the temperate zones nearest the polar regions, most of the birds are summer residents only. Few birds live all year in the polar regions. However, both the Arctic and the Antarctic have many residents during the summer.
The ranges of birds are further determined by the kinds of food and nesting places that are available. For example, fish-eating birds must live near bodies of water. Birds that nest in trees normally live only in wooded areas. Thus, most birds live not only in a particular region of the world but also in a particular type of environment, or habitat, within that region.