Birds are flying animals whose bodies are covered with FEATHERS. They are warm-blooded.

Millions of years ago the birds developed from reptiles that lived in trees. A bird's legs are covered with scales like those of a LIZARD. At first birds could only glide from tree to tree. Later birds were able to fly by flapping their wings. Eventually many kinds of birds appeared.

Kiwis, ostriches, and PENGUINS cannot fly. Swifts and albatrosses, however, can keep flying for days on end. Puffins and guillemots use their wings for swimming as well as for flying.

Most birds have their eyes on the sides of the head, so they can see almost all around them. OWLS have forward-looking eyes and can judge distances very well.

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