In most birds, 15 to 25 percent of the body is given over to flying muscles. In a hummingbird’s body, flight muscles account for 35 percent. An enormous heart constitutes up to 2.5 percent of its body weight—the largest per body weight of all vertebrates. At rest, the hummingbird pumps blood at a rate fifteen times as fast as that of a resting ostrich, and that blood is exceptionally rich in oxygen-carrying hemoglobin.
