[AS.] A winged vertebrate animal covered with feathers. Birds easily mount up into the air, their bones being of all animals largest in proportion to their weight, and the quills of their feathers filled with air. These communicate with a series of sacs or air-chambers connected with the lungs. In birds which fl y much the neck is stretched forward like a wedge, the breast bone is extended like the keel of a ship, and the wish-bone, which is the collar-bones joined into one, is much developed. In birds such as the ostrich, which run chiefly, the wish-bone does not grow. When a bird perches and bends its knees, the weight of its body pulls a large ribbon-like cord in its leg, which makes its toes clutch the perch. As it sleeps its body falls forward; and the further forward it goes, the closer do the toes cling, so that it does not fall off. The heart of a bird has four chambers, with perfect circulation. The temperature of the blood of birds is very high (1040) while that of the human body is 980. Their bodies retain this heat through the non-conducting nature of the down and feathers with which they are covered. Feathers (q. v.) are modified hairs, and are shed annually. Birds are then said to moult. The bones of the neck vary in different birds. The sparrow has nine, the swan has twenty-three.
The necks of birds are thus flexible and strong, and their heads may be turned easily, or put under their wings when they go to sleep. The back-bone of a bird is inflexible, and practically one bone. The place of teeth in animals is taken in birds by the horny growth called the beak (q.v.) Tropical birds have the most beautiful plumage. Birds usually migrate in flocks to warmer countries on the approach of winter, returning in spring, many of them being very swift in flight. All birds build nests in which their eggs are laid and their young hatched, the young being cared for in the nest until able to fly. Birds' nests are made of a great many different materials - such as straw, sticks, hay, moss, leaves, clay, wool, hair, and feathers. The outside of the nest is rough and strong, for it has to keep out the wind and the rain. The inside is generally soft and warm, like a bed. The nests of different species of birds vary greatly, from the neat little nest of the wren, with a hole in the side for an entrance, to the hole in a tree in which the woodpecker lives, the swallow's nest of dry mud, and the eagle's nest of woven sticks. Some birds build their nests in trees, some in bushes, some in hedges, some among the grass of the field, and some in the corners of windows and under the eaves of houses. Birds are classified according to their beaks and claws, which vary greatly with their habits. There are swimmers, waders, runners, scratchers, climbers, perchers, and birds of prey. (See Beak, Claw, Feather, Foot, Wing.)