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Flapping, circling, cruising, diving, landing, all part of their native technique, all come to them hardwired into the genes. The naive view of how birds fly, the common-sense view that still feels right to me, in some stubborn corner of my mind that is resistant to apparently overcomplicated science, is simply that the flapping of their wings somehow pushes the air down, and therefore them up. I knew that aircraft didn't flap their wings, though not for lack of trying among early inventors of ornithopters and other curious vehicles, but I just figured they worked in the same way that your hand is pushed upward when you hang it out a car window at speed and angle it just slightly into the wind. The wind has force, we know that from . . . well, from wind, the force of wind on trees and other objects.

The reality is more complex. If you examine a bird's wing in cross-section, it is generally flattish on the bottom and curved on the upper surface; the curvature is more pronounced in some birds than others, but it is always there, at least among those birds who still use their wings for flying, unlike ostriches, or the late-lamented dodo. That curvature, it turns out, is critical. The reason for it was worked out as long ago as the eighteenth century by Dutch-born mathematical physicist Daniel Bernoulli, although he didn't apply it to flight. He was concerned with more prosaic matters, like water pressure.

Bernoulli and Blaise Pascal pretty well invented the science of fluid mechanics—and hence the study of laminar and turbulent flow, and hence aerodynamics. Bernoulli's principle, which sounds deceptively simple, was that the pressure exerted by a moving fluid (water, or air) is a function of the speed at which the fluid is moving. This is the familiar garden hose effect that was noted in the discussion on wind force; the pressure of the same volume of water through a hose will vary depending on the diameter of the pipe—the nozzle, if you like. So if air is flowing faster on one side of an object than on the other, the faster flow will cause reduced pressure in that area, and consequently the slower side (high pressure) will be pushed toward the faster side (low pressure). Bernoulli's principle explains a good many humble phenomena. For example, it explains why a shower curtain is sucked in against the showerer—the flow of the water from the shower head decreases the pressure inside the curtain. A curveball in baseball depends on Bernoulli's principle too. The spin imparted by the pitcher causes air to move more rapidly on one side of the ball than on the other, increasing the pressure on one side, with the net effect of pushing the ball off course. Once aloft, birds obtain lift in exactly the same way. Air flows over the curved upper surface of the wing more rapidly than it does over the flatter lower surface; as per Bernoulli, the more rapid flow of air above the wing results in decreased pressure there, allowing the normal pressure beneath the wind to push upward. Birds, like planes, fly by being pushed upward by air pressure.5 They don't lift, they are pushed.

Bernoulli's principle, showing how air flowing faster over a curved surface creates lower pressure, and therefore lift. When the angle of attack is severe, cavitation is caused behind the upper surface, which exaggerates the lift force and can cause a wing to "flip."

Bird wings are rather more complicated than simple aircraft wings, which is not so surprising, given they have developed over many millennia of trial-and-error flying. Also not surprisingly— since birds probably developed from flying dinosaurs, and flying dinosaurs developed from quadrupeds—wings are really vestigial arms. Or, looked at from an avian point of view, not so much vestigial as more highly developed arms. As a consequence, bird wings, unlike those of insects or of aircraft, consist of two quite separate parts: an arm wing and a hand wing.

It is the arm wing, the part closest to the body, that yields up the conventional aerodynamic profile