The evolution of flight has endowed birds with many physical features in addition to wings and feathers. One of the requirements of heavier-than-air flying machines, birds included, is a structure that combines strength and light weight. One way this is accomplished in birds is by the fusion and elimination of some bones and the "pneumatization" (hollowing) of the remaining ones. Some of the vertebrae and some bones of the pelvic girdle of birds are fused into a single structure, as are some finger and leg bones -- all of which are separate in most vertebrates. And many tail, finger, and leg bones are missing altogether. Not only are some bones of birds, unlike ours, hollow, but many of the hollows are connected to the respiratory system. To keep the cylindrical walls of a bird's major wing bones from buckling, the bones have internal strut-like reinforcements.

The pneumatization of bird bones led to the belief that birds had skeletons that weighed proportionately less than those of mammals. Careful studies by H. D. Prange and his colleagues have shown this not to be the case. More demands are placed on a bird's skeleton than on that of a terrestrial mammal. The bird must be able to support itself either entirely by its forelimbs or entirely by its hindlimbs. It also requires a deep, solid breastbone (sternum) to which the wing muscles can be anchored. Thus, while some bones are much lighter than their mammalian counterparts, others, especially the leg bones, are heavier. Evolution has created in the avian skeleton a model of parsimony, lightening where possible, adding weight and strength where required. The results can be quite spectacular: the skeleton of a frigatebird with a seven-foot wingspan weighs less than the feathers covering it!

Not all birds have the same degree of skeletal pneumatization. To decrease their buoyancy and make diving easier, some diving birds, such as loons and auklets, have relatively solid bones. Those birds are generally less skillful fliers than ones with lighter skeletons.

Birds have found other ways to lighten the load in addition to hollowing out their bones. For instance, they keep their reproductive organs (testes, ovaries and oviducts) tiny for most of the year, greatly enlarging them only during the breeding season.

The respiratory system of birds is also adapted to the demands of flight. A bird's respiratory system is proportionately larger and much more efficient than ours -- as might be expected, since flight is a more demanding activity than walking or running. An average bird devotes about one-fifth of its body volume to its respiratory system, an average mammal only about one-twentieth. Mammalian