BIRDS. The birds form that class (Aver) of warm-blooded vertebrate animals most dis tinctive, most easily defined and most popu larly known and interesting. They are at once distinguished by their covering of feathers, which is possessed by no other sort of animal; and by the modification of their fore-limbs into instruments for flight (wings). Their aerial existence, from which few have wholly de parted, requiring great activity and exertion, has called forth a high perfection of organiza tion, especially in the respiratory and circulatory systems of the body, and has led to the charac teristic spindle-shaped form, narrowing from the full chest and shoulders toward a pointed head, which will cleave the air easily, and dimin ishing toward the rudder-like tail. The graceful form, to which the beauty of birds is largely due, has been brought about by the enlarge ment of the shoulder-girdle, and its great pec toral muscles, and by the necessity of an increased capacity of chest to contain the com paratively great heart and lungs. In birds such as ostriches, cassowaries, moss and the like, which have ceased to fly, and have de veloped very strong legs; or in those like the penguins, which have become swimmers and divers, the changes of structure are degenera tions from the type, which is a bird with powers of flight.
Character of the Feather Flight, as well as clothing, is due to the presence of the complicated horny appendages growing from the skin, called feathers, which are peculiar to the class. Their structure is de
scribed under FEATHERS. Those of the body are usually small, grow in certain definite tracts (see PTERYIAGRAPHY ), varying in the different groups, and form a close jacket, not easily pervi ous to moisture and a poor conductor of heat, thus conserving the vital warmth and protect ing the body against sudden changes of temperature. It is shed (molted) and renewed semi-annually. This body-coat is ordinarily nearly uniform in length and character, but often is varied by ornamental plumes, erectile crests, ruffs, and other modifications, such as are seen in birds of paradise, herons and many others. The feathers are also variously colored in patterns varying with the groups and more minutely with the species, whereby they may recognize each other and be distinguished by us. These colors are usually those of pig ments incorporated in the web of the feather itself, but may be due to minute scales on the surface, which break up the light, giving it an iridescent or metallic sheen, conspicuous in hummingbirds and in certain pheasants. The plumage often varies, according to age, sex, sea son, or all three conditions; and these colors play an important part in bird-life. See COLORA