Bird, animal adapted for flight and unique in its body covering of feathers. There are more than 8,500 species. Birds are warm-blooded descendants of reptiles of the dinosaur group. They developed feathers from scales (still evident on their legs) and became two-legged as their forelimbs became wings. Their teeth disappeared, replaced by a horny bill used for feeding and performing complicated tasks, such as nest-building. The bird's body has been adapted for flight. The feathers, an efficient and light body covering, streamline the body and extend the flight surfaces: wings and tail. The skeleton is light, and the bones are hollow. Large breast muscles provide power for flight. The heaviest flying bird is the trumpeter swan of North America, which can weigh up to 38 lb (17 kg). At the other end of the scale, the bee hummingbird is about 2.5 in (6 cm) long and weighs 1/10 oz (3 g). Flightless birds can be much larger. The ostrich stands up to 8 ft (2 1/2 m) and can weigh 300 lb (136 kg). Flightless birds are mainly adapted for running or swimming. Runners have strong legs, like the ostrich; swimmers have wings that are modified as flippers, as in penguins. But some flying birds are also powerful runners, or swimmers. All birds reproduce by laying eggs that must be kept warm for correct development.
In 1997 fossils were found in Patagonia, which belonged to a previously unknown dinosaur variety (IUnenlagia comahuensis). Scientists put that this variety is the missing link between birds and dinosaurs. In 1998 scientists presented the Rahona ostromi, a primitive bird with claws that resemble those of carnivorous dinosaurs.