Birds belong to the class Aves, which contains dozens of orders. Birds are warm-blooded vertebrates with wings, feathers, and light hollow bones. The vast majority of birds are capable of flight. According to the U.S. Fish and Wildlife Service (FWS), more than 800 species of birds spend all or part of their lives in the United States; more than 9,000 species of birds have been identified around the world.
In addition to taxonomy, birds are broadly classified by their physical characteristics (such as feet or beak structure), eating habits, primary habitats, or migratory habits. For example, raptors or birds of prey have curved beaks and talons well suited for catching prey. This category includes the eagles, vultures, hawks, buzzards, and owls. Perching birds have a unique foot structure with three toes in front and one large flexible toe to the rear. Ducks and geese are known as open-water or swimming birds and have webbed feet. Habitat categories include the sea birds, shore birds, and arboreal (tree-dwelling) birds. Some birds migrate over long distances and others, like turkey and quail, do not migrate at all.
Scientists believe that more than 100 bird species have gone extinct during the course of human history. Bird species have died out because of habitat destruction, hunting and collection, pollution, and predation by nonnative species. The extinction rate of bird species is alarming not only because of the irrevocable loss of each species but also because of implications for the health of entire ecosystems.