A bird is a member of the class Aves of vertebrates distinguished by having feathers, two legs, and two wings. Birds are warm-blooded animals, and their young hatch from eggs. According to one theory, modern birds are descended from the dinosaurs. About 8,600 extant species are known.
Many of the bones in a bird's skeleton are hollow to reduce the weight of the animal for flying. Like humans, a bird has ribs. These form a protective cage around the heart and lungs. The backbone and shoulder and hip girdles are adapted so that the weight of the body is carried on the wings in flying and on the legs in walking. The large flight muscles may account for one-fifth of the total body weight and are attached to a large, keeled breastbone and the upper part of the wing.
A bird has large air sacs in the spaces in its body and even in the hollow bones. They are filled with the air that passes through the lungs when the bird breathes and are emptied through the lungs when the bird breathes out. In land animals, air flow is bidirectional: air travels through a network of channels, and stops at the alveoli where oxygen-carbon dioxide exchange takes place. Used air, containing carbon dioxide as a waste product, follows a reverse course in leaving the lung and is discharged through the windpipe. In birds, however, air flow is unidirectional. New air comes in one end, and the used air goes out the other end. This provides an uninterrupted supply of oxygen, necessary to meet a bird's high level of energy consumption.
Birds have no teeth but a tough, horny beak. The food is not chewed but is stored in the crop after it is swallowed and is later ground up in the muscular gizzard which may contain stones.
Varieties of birds
The size of birds ranges from the bee hummingbird, 6.4 centimeters (2.5 inches) to the wandering albatross, with a wingspan of 3.5 meters (11.5 feet). The 2.5-meter (8-foot) tall ostrich is the largest of living birds, but several extinct flightless birds were even bigger. Of the 27 orders of birds, the perching birds (Passeriformes) include more species than all others combined.
There are several groups of flightless land birds, including the ostrich, rhea, emu, cassowary, kiwi, and penguin.