Why do birds' wings help them fly?

A bird's flight obeys the same physics as an airplane's. Seen in profile, a bird's wing has a rounded leading edge, a top surface more curved than the bottom one, and a thin trailing edge: exactly an airfoil. As the bird moves forward, the wing cuts through the air at a certain angle of attack and forces the airflow to curve downward. By Newton's third law, the air deflected downward pushes the wing upward; at the same time, the accelerated flow over the top surface leaves a lower pressure there than on the bottom. Both descriptions are sides of the same phenomenon, lift, and together they explain why a gull can hold itself up gliding without flapping its wings.

An airplane separates the two functions: the wing lifts and the engine pushes. The bird fuses them in the flap. On the downstroke, the outer part of the wing (the "hand", with the primary feathers) tilts and acts like a propeller, pushing air backward and downward, producing thrust in addition to lift. On the upstroke, the wing folds partially and the primaries separate like slats to let air through and reduce drag. Large birds, such as storks or vultures, minimize flapping and use thermal currents to gain altitude by circling; small ones, with short, rounded wings, flap quickly to maneuver through vegetation.

Flying requires lift to equal weight, so evolution has lightened everything it could. Birds' bones are hollow and reinforced with internal struts, the beak replaces teeth, and air sacs extend the lungs into the skeleton. Feathers, too, are extraordinarily efficient structures: rigid in the shaft, flexible in the barbs, and able to hook together to form a continuous surface that can be repaired with the beak. The bird adjusts the shape of its wing instant to instant, changing sweep, twist, and camber, and uses the tail as a stabilizer and brake. An airplane mimics that control with flaps, ailerons, and rudders; the bird has it built into its muscles and tendons.

Pioneers from Otto Lilienthal to the Wright brothers studied bird flight to understand how balance is maintained and how a lifting surface is controlled. The wing warping of the 1903 Flyer came from observing how vultures twist their wingtips to turn. Today, the winglets on commercial airplanes mimic the spread primaries of raptors to reduce induced drag at the wingtip.