Hovering Flight
photographs of a White-tailed Kite and Anna's Hummingbird by Rohan Kamath	Some birds, such as kestrels, remain motionless "wind hovering" above a point on the ground by flying into the wind at a speed equal to that of the wind, and other birds hover momentarily while foraging. Hummingbirds, on the other hand, are able to remain in the same place in still air as long as they wish -- they are true hoverers. A hovering hummer keeps its body at about a 45 degree angle to the ground and moves its wings in more or less a figure-eight pattern, with the "eight" lying on its side. Hummers have an extremely mobile shoulder joint, permitting them to twist the wing in such a way as to generate lift on both the backward and the forward strokes. The front edge of the wing leads on both strokes, and on the backstroke it is the underside of the feathers that face upward, the shoulder rotation having, in effect, turned the wing upside down. In each stroke the bird is able to make use of some of the energy transferred into the motion of air on the previous sweep of its wings. For instance, on the forward stroke the airspeed of the wing is increased because it is traveling through air pushed toward the rear of the bird by the previous backstroke. The direction of thrust changes between the forward and backward strokes, so that they cancel each other out. Since the wings beat more than 20 times per second (sometimes as rapidly as 80 beats per second), inertia holds the bird's body essentially stationary. This system makes hummingbirds extremely maneuverable and permits them to hover while they extract nectar from flowers that might otherwise be inaccessible. But the hovering flight is quite expensive: about 30 percent of the total body weight of
hummingbirds is invested in the breast muscles (which power the wings), whereas other strong-flying birds have about 20 percent, and weak fliers may have only about 15 percent. The hummers would require an even greater investment in muscle if it were not for the way that their wings are always moving through air accelerated by the previous wing stroke.
SEE: Adaptations for Flight; Wing Shapes and Flight. Copyright ® 1988 by Paul R. Ehrlich, David S. Dobkin, and Darryl Wheye.