The class of animals consisting of the birds. Modern birds are characterized by being feathered, warm-blooded (homeothermic), and bipedal (two-legged) with the forelimb modified into a wing which, together with the tail feathers attached to the short tail, forms the flight mechanism, and by having a very high metabolic rate. Such a characterization, however, as with any group of vertebrates, holds for the living forms and most fossil members of this class, but is blurred by the early fossil record, which contains species with characteristics closer to those of the reptilian ancestors of birds.

The feathers of birds are lightweight modifications of the outer skin possessing remarkable aerodynamic qualities. They serve not only as surfaces to generate lift and thrust, and as a streamlined outer surface of the body, but also as insulation to maintain high body temperatures. In addition, birds have lightweight, hollow bones; a well-developed air-sac system and flow-through lungs; a wishbone or furcula (fused clavicles); and a hand reduced to three digits (comparable to digits 2, 3, and 4 of the human hand).

Birds have most likely evolved from an ancestor within the large group of ancient diapsid reptiles known as archosaurs (including alligators, snakes and lizards, and dinosaurs, among others). However, debate still centers on whether birds are derived from a basal archosaurian stock or arose later directly from the later and more derived theropod dinosaurs (carnivores such as Allosaurus and Velociraptor). The latter theory gained support by the discovery of fossils from the end of the Mesozoic Era (“Age of Reptiles”) that some researchers reported to be feathered dinosaurs; however, other researchers think they were birds that had already became secondarily flightless and represent “Mesozoic kiwis.”