When a bird is gliding, it flies the same way as an airplane. The wings are held out to the side of the body and do not flap. As the wings move through the air, they are held at a slight angle, which deflects the air gently downward. This causes air pressure to build up beneath the wings, while the pressure above the wings is reduced. The difference in pressure is lift, a force that acts roughly perpendicular to the wing surface and keeps the bird from falling.

In gliding flight, a bird's wings deflect air downward, causing a lift force that holds the bird up in the air.

There is also air resistance or drag on the body and wings of the bird. This force would eventually cause the bird to slow down, and then it wouldn't have enough speed to fly. To make up for this, the bird can lean forward a little and go into a shallow dive. That way, the lift force produced by the wings is angled forward slightly and helps the bird speed up. Really what the bird is doing here is giving up some height in exchange for increased speed. (To put it another way, it is converting its gravitational potential energy into kinetic energy.) The bird must always lose altitude, relative to the surrounding air, if it is to maintain the forward speed that it needs to keep flying.

By tilting forward and going into a slight dive, the bird can maintain forward speed.

Next, read about soaring flight.

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