Flapping flight—the way most birds fly—is more demanding still. Achieved by flexing wings at joints in wrist, elbow, and shoulder, it is powered by extraordinarily strong breast muscles. The wings move forward in a downward arc, propelling the bird forward and up. It is similar to the oarsman’s power stroke or the action of a swimmer doing the butterfly. Movement then flows into the upward stroke, a recovery stroke, to start the process anew.
