Birds make up the class Aves. The earliest known fossil bird is classified as Archaeopteryx lithographica. Bird, common name for any member of the class of vertebrates that contains animals with feathers. All adult birds have feathers, although some species, such as pelicans, kingfishers, woodpeckers, and jays, are completely naked when hatched.

The earliest known fossils identified as birds link their ancestry to reptiles, possibly small dinosaurs of the Triassic period (245 million to 208 million years ago). The earliest known fossil bird is Archaeopteryx, about the size of a small pigeon, of which six complete or partial specimens-and an isolated feather-have been found in the Solnhofen limestone beds in Germany; all date from the Late Jurassic period (157 million to 145 million years ago). The species is a mixture of bird-like and dinosaurian anatomical characteristics. If the original skeletons had not clearly shown imprints of feathers exactly like those of modern birds, the fossils might have been classified as small, somewhat peculiar dinosaurs. Archaeopteryx had teeth, which are lacking in all modern birds. Feathers undoubtedly evolved from reptilian scales, but lacking fossil evidence, the mode of transition from scale to feather remains a subject for conjecture. Some skeletal features of this earliest known bird are typical of modern birds, however, and are not known from reptiles.

"Bird," Microsoft(R) Encarta(R) 97 Encyclopedia. (c) 1993-1996 Microsoft Corporation. All rights reserved.