A bird's wing is so shaped that the air must travel farther over the top of the wing than underneath it. Hence, the air above the wing travels faster to 'catch up,' as it were.

Due to increased speed, the air above the wing is "thinner" than the air below. The compressed "thicker" air under the wing exerts greater pressure and pushes the bird up, providing the needed lift. Something similar occurs when you drink with a straw. As you suck on the straw, you are thinning out the air inside the straw. The normal air outside is then "thicker" and pushes the liquid up the straw.

The air striking the underside of the bird's wing also tends to lift it. At the same time, however, the creature has to use some of its strength to overcome the air's dragging effect.

To get airborne, a bird usually jumps into the air, flapping its wings. At first it may seem that the bird just flaps them up and down. But closer investigation reveals that this flapping flight is far more complex. The bird pulls its wings down and back with the feathers tightly closed and wings outstretched, thereby pushing as much air as possible. Then it pulls the wings forward and up with the feathers separated to allow the air to pass through. The wings are also pulled in close to the body so that there is minimum air resistance.

Wing movement provides lift as well as the propulsion needed to overcome "drag" and to gain speed. The bird's wing movement might be compared to a swimmer's doing the "butterfly stroke." His arms rotate around his shoulder joint, as he throws them forward through the air and then pulls them back through the water.