A class (Aves) of endothermic (see endotherm) vertebrates that are adapted for flight, bipedal walking or running, and, in some species, swimming on or below the surface of water; flightless species (ratites) are believed to have diverged from flying birds and subsequently to have lost their adaptations for flight. The bones are light and often tubular, sometimes strengthened by internal struts, the body is covered with feathers, the forelimbs are modified to form wings, and the jaws, which are usually long and slender, lack teeth and support a horny bill. Many of the bones contain extensions of the air sacs. Except in ratites, which lack it, a keel on the sternum provides attachment for powerful flight muscles. Birds are descended from archosaurian reptiles and retain a number of reptilian characteristics (e.g. the arrangement of the parts of the skull and the scaly covering of the legs and feet; feathers are also derived from scales). The skin is thin and lacks sweat glands. There are about 8700 species, with a worldwide distribution.