bones are hollow. Even their skulls are scaffolded with passageways for air. Their feather shafts are hollow, and the feathers themselves, like strips of Velcro, are interlocking barbules for catching air. Their bodies are filled with air sacs, which originate in, and function, in part, as extensions of the lungs. No fewer than nine of these filmy bladders fill the tiny body of a hummingbird: one pair in the chest cavity; another under each shoulder blade; another pair in the abdomen; one under each wing; and one along the neck.
