A Lightweight Skeleton

The fact that a pelican approximately 5 feet long weighing nearly 20 pounds has a skeleton weighing only 23 ounces indicates how perfectly a birds skeleton is adapted to its capacity for flight. The reason the skeleton is so lightweight is that many bones in a bird's skeleton are hallow. The hollow bones are honeycombed with air spaces and strengthened by crisscrossing struts. The number of hollow bones varies from species to species, though large gliding and soaring birds tend to have the most. In general, the more efficient fliers seem to have more bones that are hollow.

A bird's streamlining for flight is perhaps best exemplified in the evolution of the skull, which is composed mainly of thin, hollow bones. A bird's skull is extremely light in proportion to the rest of its body due to elimination of a heavy jaw, jaw muscles, and teeth; the job of chewing has largely replaced by the gizzard. The skull usually represents less than 1 percent of a bird's total body weight.