A large bird soars in the distance off toward the horizon. It’s too far to see clearly. You might wonder if it’s a red-tailed hawk, a turkey vulture or even a golden eagle. Keep watching. Is it rock steady on the wing or does it rock and roll, tipping a bit back and forth as the unstable air disturbs its flight? If it wobbles, it is undoubtedly a turkey vulture.
A vulture doesn’t hold its wings perfectly flat. Rather the wings are raised a bit above the body forming a shallow “V.” This V-shape is called a “dihedral,” and it is the vulture’s secret to nearly effortless flight. Most hawks and eagles, in contrast, hold their wings flat.
A wing held parallel to the ground produces lift in an upward direction. This is the normal position we think of for a gliding bird in flight. No surprise here. A wing held vertical with the tip pointing straight up produces “lift” sideways but none in an upward direction to keep a bird aloft. No real surprise here either. A wing held somewhere in between produces an intermediate amount of upward lift.
Vultures do the “something in between.” The “V” of a vulture’s wings means it is not producing as much lift in an upward direction as it could if instead it held its wings perfectly flat. This seems inefficient. Why not maximize lift? Flying is hard enough without sacrificing efficiency.
Although we can’t see it, the atmosphere is always in motion, swirling and blowing with eddies and gusts. Heated summer air is especially turbulent. Flying birds must constantly adjust to this turbulence.
A puff of air from the side might knock a vulture off its flight path, tipping the bird sideways. One wing goes up a bit and the other down. Not to worry. The wing that is tipped up, let’s say the left wing, is a little closer to vertical resulting in a little less lift in an upward direction. The right wing of the “V” shape is now a little closer to parallel to the ground increasing its lift upward slightly. With greater lift on the right wing and less lift on the left wing, the bird tends to return to level flight — all without moving muscle or wing. This produces the wobble we observe.
A red-tailed hawk flying on flat wings loses lift on both wings if knocked off balance by a gust of wind. To right itself, it must adjust its wings and/or tail or even flap. All this takes energy. It seems the vulture has a pretty good solution to soaring and gliding in turbulent air. A turkey vulture is the avian equivalent of a couch potato. It works no harder than absolutely necessary.
The vulture isn’t the only bird to discover this secret. Northern harriers also fly with a dihedral. Instead of flying high like a vulture, they fly closer to the ground, where they glide low over grassland and marsh hoping to surprise prey. They wobble, too. There are others. Even a great many airplanes have a dihedral for greater stability.
Now you, too, can identify a turkey vulture in flight from more than a half-mile away.
Stewart Janes is a biology professor at Southern Oregon University. He can be reached at firstname.lastname@example.org.