Bird Flight (2024)

In the Gliding page, we saw how a bird can move through the air without flapping its wings. Unfortunately, gliding flight always results in a bird moving downward through the air. Soaring flight is a special kind of glide, in which the bird flies in a rising air current. Because the air is rising, the bird can maintain its height relative to the ground.

Soaring flight can only occur at special places and times. For example, warm air heated by the sun can rise up from the hot ground and into the sky. This rising air current is called a thermal. Thermals often rise up along the slope of a hill, but they can also form over flat ground. As the air rises, it also expands and cools. (This results from the lower pressure at altitude.) Eventually, the water vapor in the thermal may become cold enough to condense, forming the tiny droplets of liquid water that make up a cloud. When you have a sunny day with puffy clouds, it's probably a good day for thermals, and you may see hawks or vultures soaring overhead.

Soaring birds can also find rising air in places where the wind is forced to flow up the side of a hill. Long ridges produce the best lift. Smaller, isolated hills don't produce as much lift because the air can flow around the sides of the hill instead of going over the top. The amount of lift and where it is found will depend on the speed and direction of the wind, as well as the shape of the land. Smaller objects such as trees or houses produce ridge lift too, though it may not be enough to keep a bird in the air.

Finally, there is one more soaring technique, called dynamic soaring. Dynamic soaring does not rely on rising air currents. Instead, it uses the difference in wind speed between the ground and higher up. Here's how it works:

• First, the bird must climb up into the faster airflow, ten or twenty meters above the ground. It climbs facing the wind, so it receives the benefit of a constantly increasing air speed as it goes up higher. It may appear that the bird is slowing down, but its speed relative to the surrounding air is actually increasing.
• Then the bird makes a turn and heads back downwind. From the ground, it seems that the bird has suddenly gained a tremendous amount of speed, because now the bird is flying along with the wind.
• Then the bird dives down into the lower, slower air. Again its air speed increases, this time assisted by gravity.
• The bird turns back upwind and repeats the cycle.

Dynamic soaring works best in places where there are no upwind obstructions that would block the wind and cause turbulence. This means flat, open ground, over the ocean, or at the top of a ridge. The albatross uses this type of soaring to support its multi-year voyages at sea.

Next, read about flapping-wing flight.

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