The birds are the acknowledged masters of the air. Their abundance and diversity, both past and present (with some 166 families of living birds), exceeds that of any other flying creature. Their success lies in the development of a structure unique in the animal world — feathers. These aerodynamic structures, developed from reptilian scales, were the ideal innovation for flight. They are lightweight and easily replaced if damaged, in contrast to the vulnerable skin-wings of pterosaurs and bats.
By transforming the forelimbs of their reptilian ancestors into feathered wings, birds acquired the enlarged surface area needed to support the body in the air. The wings could be folded away into a compact shape when the bird landed. The modified forelimbs could therefore be wholly devoted to the needs of flight, leaving the hindlimbs free to become adapted for walking upright.
- The power for active, flapping flight comes from large muscles that make up 15 to 30 percent of a bird's total body weight. They stretch downward from the wings and attach to the pectoral, or shoulder, girdle. The sternum, or breast bone, is greatly enlarged, and bears a prominent keel (carina) down the middle; both surfaces provide a large area to which the wing muscles can attach (see p. 173). They also attach to the fused clavicles, or collar bones, which form the wishbone (furcula), and to a membrane that connects the wishbone to the sternum.
Archaeopteryx, the earliest-known bird, appeared in the Late Jurassic. The Cretaceous period saw an explosive evolution of the birds, and most modern groups had appeared by the end of that period or by the Eocene.
Since birds are rarely fossilized, this evolution chart is based on recent studies, carried out in the USA by Sibley and his colleagues, on the genetic material of modern birds. Only a proportion of the 27 or so orders of modern birds are shown here. Relatively few groups have become extinct over their 140 million years of evolution. (For key to silhouettes, see p. 312.)
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