Adaptation for flight is highly developed in birds and insects. The bat is the only mammal that accomplishes true flight; flying squirrels glide rather than fly, as do flying fish and flying lizards. Birds fly by means of the predominantly up-and-down motion of their wings. The flapping motion is not, however, straight up and down but semicircular, the wings generally moving backward on the upstroke and forward on the downstroke. That motion pushes air downward and to the rear, creating a lift and forward thrust. The leading edge of the slightly concave wings is rather sharp, and the feathers are small and close-fitting, so that a streamlined surface meets the air. On the trailing edge of each wing the interlocking of the larger feathers forms a surface that acts somewhat like the ailerons, or movable airfoils, of an airplane. In wing motion, the leading edge is twisted so as to be lower than the trailing edge in the downward stroke and above the trailing edge in the upward stroke.
Besides flapping, some birds also use gliding and soaring techniques in flight. In gliding, a bird holds its outstretched wings relatively still and relies on its momentum to keep it aloft for short distances. In soaring, a bird uses rising warm air currents to give it lift.
The form and size of wings vary in different birds. In woodland birds the wings are somewhat rounded and have a relatively broad surface area. Birds with well-developed gliding ability, such as gannets and gulls, usually have narrow, pointed wings. Especially noted for their soaring power are eagles, vultures, crows, and some hawks. In soaring flight the feathers on the wings of these birds separate at the tips, resembling opened fingers against the sky. It is thought that this movement diverts the airstream over the wing and aids the bird in turning, banking, and wheeling. There is disagreement as to the maximum speeds achieved by birds in flight. While the flight speeds of most birds range from 10 to 60 mi (16–100 km) per hr, some have been recorded at speeds reaching 70 mi (110 km) per hr, for long distances and near 100 mi (160 km) per hr, for short flights. In a stoop, falcons can reach faster speeds.
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