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The Complete Guide to Birds
Birds are bipedal, warm-blooded, egg-laying vertebrates characterized primarily by feathers, forelimbs modified as wings, and hollow bones. Birds range in size from the tiny hummingbirds to the huge Ostrich and Emu. Depending on taxonomic viewpoint, there are between about 8,800-9,800 living bird species (and about 120-130 which have become extinct in the span of human history) in the world, making them the most diverse class of terrestrial vertebrates.
Birds are a very differentiated class, with some feeding on nectar, seeds, insects, rodents, fish, carrion, or other birds. Most birds are diurnal, or active during the day. Some birds, such as the owls and nightjars, are nocturnal or crepuscular (active during twilight hours). Many birds migrate long distances to utilise optimum habitats (e.g., Arctic Tern) while others spend almost all their time at sea (e.g. the Wandering Albatross). Some, such as frigatebirds, stay aloft for days at a time, even sleeping on the wing.
