Most birds can fly, and all are ultimately descended from ancestors that could fly, although there are many extinct flightless species. The bodies of birds are therefore highly modified for efficiency in flight. The digital and wrist bones of the forelimb are extensively fused to form a rigid support for the large flight feathers of the wing. Fusing of bones for strength and lightness is also found in the skull and pelvic girdle. Many bones of adult birds are hollow rather than filled with marrow, and are connected to a system of air sacs dispersed through the body. The sternum, or breast bone, of most birds is relatively large and bears a central ridge, or keel, known as the carina. The sternum and carina support some of the major muscles used in flying. Flightless (ratite) birds—including the ostrich, the kiwi, and their relatives—do not have a carina and the sternum is reduced in size. The wing structure of most flying birds is the same but there are differences in flying styles. Larger flying birds, such as buzzards, vultures, albatrosses and eagles, spend much of their time in the air gliding or soaring. They use their broad wings with splayed feathers to glide on strong winds or “rest” on rising warm air. Most smaller birds have to flap their wings to sustain their flight.
The jaws of modern birds are extended as toothless bills or beaks, covered with a layer called the ramphotheca, which can be hard, as in most birds, or leathery, as in ducks. The lack of teeth lessens the weight of the skull.