The Distribution of Birds

Every species of bird has its own range-that is, a particular part of the world in which all the members of the species normally live. Some birds have a broad range. The osprey and common barn owl, for example, live on every continent except Antarctica. However, no species of bird is found in every part of the world, and many species have an extremely limited range. For example, a species called the Whitehead's broadbill lives only in a small, mountainous area of northern Borneo.

Oceans and continents strongly influence the distribution of various species of birds. Most birds cannot make long ocean flights. Widely separated continents, such as Africa and North America, therefore have different kinds of birds. However, people have transported many species overseas, and some of these birds have become adapted to their new environment.

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.
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