Why Birds Can Fly Birds are well-adapted to flight. Their bodies have adapted by becoming very light yet strong. Birds are lighter than other animals. Compared to a reptile or mammal of a similar size, birds are much lighter.
Bird bone weighs about half as much as mammal bone because the bird bone is hollow.
The bones of the wing and leg are long hollow tubes that are supported by many tiny cross braces. These maintain the strength of the bone without the additional weight. In the same manner the bird's skull consists of two thin layers of bone that are connected by bone braces. Another way to maintain strength without adding weight is for two or more bones to be fused together. Such bones have permanently grown together, creating one large bone.