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Some land birds, such as vultures and certain hawks, sustain flight for long periods without flapping their wings. They take advantage of updrafts produced when the wind blows over hills and mountain ridges or make use of rising columns of warm air called "thermals." Vultures stay within thermals by flying slowly in tight circles. They have short, broad wings and a low wing loading (ratio of bird weight to wing area) that allows them to remain aloft and to be highly maneuverable at slow speeds. They also have a low aspect ratio (ratio of length to width of the wing), something that is dictated by their takeoff requirements. Low-aspect-ratio wings generally produce a lot of drag -- that is, resistance from the air through which they are moving. Air from high-pressure areas beneath the wings tends to flow over the wingtips into the low-pressure areas above the wings. That flow produces wingtip turbulence, drag-creating disturbances of the smooth flow of air. A low-aspect-ratio wing, important for maneuvering, nevertheless creates a great deal of drag, something that is very undesirable in a soaring bird. Vultures alleviate this problem slightly by flying with their primary feathers extended, creating slots between them. Each primary serves as an individual high-aspect-ratio wing, reducing wingtip turbulence and lowering the stalling speed of the wing so that the bird can remain aloft at a slower speed. This helps vultures to circle perpetually in thermals, maintaining thrust by gliding downward, but staying aloft by sinking at a rate slower than the hot air is rising.
It has been possible to measure a vulture's rate of sink by flying in aircraft in close formation with them. Turkey Vultures have a minimum sink rate of 2 feet per second, while Black Vultures have a minimum rate of 2.6 feet per second. Black Vultures, therefore, need stronger thermals than Turkey Vultures, which helps to explain why they are restricted to the southern United States while Turkey Vultures can penetrate the relatively cool climes of southern Canada. Albatrosses and other seabirds such as shearwaters and petrels also soar. But their techniques are different from those of vultures. Albatrosses have long, slender wings with a high aspect ratio. They have the longest wings of any birds; the wingspan of the Wandering Albatross is in the vicinity of 10 feet. The high-aspect-ratio wings of soaring seabirds minimize drag, since the amount of wingtip is small in comparison with the length of the wing. The wing loading of albatrosses is very high also. Indeed, it is thought that albatrosses are close to the structural limits of wing length and wing loading. Albatrosses and other soaring seabirds use their high wing loading and high-aspect-ratio wings to take advantage of the slope lift, updrafts created on the windward slopes of waves in the same manner they are created on mountain ridges. Albatrosses are able to proceed upwind by zigzagging along in the slope lift, and can even soar in windless conditions if there are waves. The waves push air upward as they move, and the albatrosses stay in that rising air. Seabirds can also extract some energy from the altitudinal gradient in the wind, which is slowed by friction near the water and increases in speed with height above its surface. That process has been called "dynamic soaring," but recent work by a leading authority on bird flight, Colin Pennycuick, indicates that slope soarers gain relatively little energy in that way. For instance, in typical wind conditions in the South Atlantic, dynamic soaring would permit albatrosses to rise about 10 feet above the surface, but they are regularly observed to soar to near 50 feet. SEE: Wing Shapes and Flight. Copyright ® 1988 by Paul R. Ehrlich, David S. Dobkin, and Darryl Wheye. |
