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 8,800-10,200
living bird species (and about 120-130 that 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.
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