Birds have thin, hollow bones to lighten their weight and make it easier for them to fly. They also have a lower number of bones than other animals because the bones have fused together.
In some bird species, the skeleton actually weighs less than the bird's feathers. The hollow, or pneumatic, bones are filled with air, allowing the bird to defy gravity when it flies. Other aspects of a bird's skeleton also help in flight. For example, the bird has a fused backbone that helps it stay straight while it is in the air. Additionally, a bird's ulna has a series of bumps to which secondary flight feathers attach, providing structural support that permits the bird to catch the air and take off.