A bird's plumage, while contributing to structural integrity and participating in aerial locomotion, completely obscures a bird's internal activity from human view, greatly impeding our attempts to understand birds as functioning animals. Plumage has even made it difficult to describe bird movement. When describing the movement of mammals, writers turn to well-worn clichés like "grace in motion." A mammal's rippling muscles slide smoothly over one another in eye-catching ways. With a mammal, whether a mouse or hippopotamus, we recognize that the underlying body parts are similar to our own and we know these parts will act predictably. Not so with a bird. For centuries, we knew little more about a bird's movement than that it was a mystery that seemed to be based on the flapping of wings.