What Is a Bird?
Just exactly what is a bird? Perhaps you would say that a bird is an animal that flies. But butterflies, which are insects, and bats, which are mammals, also fly. Some birds, on the other hand, do not fly at all. The ostrich, the emu, and the kiwi run very fast. The penguin swims with his short paddlelike wings. None of them can fly.  

All birds, however, have feathers, which no other animal has. Feathers then, and not flying, make a bird different from other animals. Birds are feathered, warm-blooded animals with backbones. They have two legs. Whether they fly or not, all have a pair of wings corresponding to the arms or the front legs of many other animals. A beak takes the place of a jaw with teeth. All birds lay eggs. Most of them build a nest in which they care for the eggs and the young birds.

bird (burd) n. [[ME bird, brid < OE bridd, bird, orig., young bird]]
1 any of a class (Aves) of warmblooded, two-legged, egg-laying vertebrates with feathers and wings
2 a small game bird: distinguished from WATERFOWL
3 a clay pigeon in trapshooting
4 a shuttlecock
5 [Colloq.] a person, esp. a mildly eccentric one
6 [Slang] a sound of disapproval made by vibrating the lips
7 [Slang] a rocket or guided missile
8 [Brit. Slang] a young woman --vi.
9 to shoot or catch birds
10 to engage in bird-watching –
birds of a feather people with the same characteristics or tastes --eat like a bird to eat very little food --for the birds [Slang] ridiculous, foolish, worthless, useless, etc. --the birds and the bees [Colloq.] the basic facts about sexual matters. (in some bird species both sex looks alike.)     

Birds are classified under various families and Scientific study of bird called Ornithology