Gliding

Gliding uses less energy than flapping. The bird stretches its wings and coasts through the air, as gulls do when return­ing to their cliff-top roosts at sunset. Gliding birds can also soar (rise high) by seeking out rising currents of air, or thermals. Vultures using thermals circle effortlessly for hours at a time, and they can soar very high-one African vulture collided with an airplane at 37,000 feet (11,280 meters). Birds soar, too, on uplifts of air pushed up by a hillside or cliff or formed where blocks of warm and cold air meet.

Seabirds, such as gulls, make use of the different wind speeds in the air above the ocean. Over the sea, wind speed is reduced close to the water because of friction with the waves. The fastest wind speeds are usually between 50 and 100 feet (15 and 30 meters) above the waves. Some seabirds, such as fulmars, use these faster-moving winds to pick up speed and then glide down with the wind behind them to skim low above the water. Their momentum takes them up again into the wind. For such maneuvers, long wings provide stability as well as high speed.

Hovering

Some birds can hover in the air, using their wings and tail to maintain posi­tion. They hover usually to feed or spot prey below. Kestrels and terns can do this, but the hovering champion is the hummingbird, which not only hovers, but is the only bird able to fly backward.

Hummingbirds hover by beating their wings backward and forward so fast that they produce lift without propulsion. In this way, the bird stays in one place when feeding from flowers. Kestrels use a slightly different hovering technique. This bird flies into the wind at exactly the same speed as the wind, so that one force balances the other.