All birds belong to a class of warm-blooded animals with feathers.
All adult birds have feathers, although some kinds, such as pelicans, kingfishers, woodpeckers, and jays are completely naked when hatched. Most living birds can fly and their bodies are highly adapted for efficiency in flight. The forelimb bones are fused to form a rigid support for the large flight feathers on their wings. Many bones of adult birds are hollow and connected to a system of air sacs dispersed through the body. This unique bone structure provides birds with a light, yet strong skeletal system.