It was March, just before evening, and Sandhill Cranes were rising high above the Platte River in Nebraska, soaring in tight circles, and then dropping vertically into the shallow water.
The landings were the most spectacular I’ve ever seen. With wings outstretched and legs dangling, the cranes looked more like parachuting humans than birds. The cranes seldom moved their wings during the long descent, but a flurry of beating near the water permitted them to touch down slowly. The Platte attracts cranes because it’s “a mile wide and a foot deep.” They have stopped here for centuries.
To consider landing, let’s start with flight. A bird requires air to flow smoothly over the top of its wings to generate lift. When a bird decides to land, it must reduce speed, cancel lift, and ultimately come to a stop. While there are exceptions, the general pattern is that, just prior to touchdown, the bird tilts backward, raising the front of its wings, thereby increasing the so-called angle of attack. This slows the speed and, if the angle is great enough, disrupts the flow of air over the wings, creating turbulent eddies that cancel lift and cause stalling. Landing is a controlled stall.
If the wings are at a stalling angle, but lift is still needed, the bird will make use of a clever anti-stall mechanism called the alula. Located on the leading edge of the wing just beyond the wrist, the alula consists of a single bone, to which three feathers known as alular quills are attached in an overlapping row.
To activate the alula, a bird elevates it slightly, creating space between the quills and wing. Air rushes both under and over the alula and then combines to flow smoothly over the wings, maintaining lift even when the wings are at a stalling angle.
If the speed before touchdown is too fast, a bird has a way to brake quickly: It tilts back more, so its wings are nearly vertical, and beats them forward strongly, in a horizontal plane.
When airliners land or take off, the wings are at a steep angle of attack and stalling is of great concern. The narrow leading edge of the wing, the slat, is moveable and analogous to the avian alula. The slat is moved forward and downward, creating an opening. As with the avian alula, air rushes under and over the slat and continues smoothly over the top of the wing, maintaining lift.
Birds that land on trees, the ground, or other hard surfaces have to reduce their speed to zero abruptly. Birds cushion themselves from such quick stops by collapsing their extended legs, which function like magic springs.
Landing styles depend largely on a combination of body mass and wing size and shape. For most birds, these factors are a compromise between what is ideal for their lifestyle (that is, how they get food) and efficient flight.
Consider the Great Blue Heron and Common Loon. They have similar body sizes and weights, but their wings are dramatically different. The heron has large, broad wings, while the loon has small wings that appear almost too small for the job. Why the difference?
The Great Blue feeds in shallow water that is often marshy or wooded. It has to make soft, vertical landings as well as near-vertical takeoffs. Big wings are just the ticket, since they generate enough lift for herons to drop slowly.
The Common Loon, on the other hand, dives and swims underwater, where it feeds largely on fish. If it had the heron’s large wings, its underwater swimming would be abysmal and it would starve. Fortunately, its wings favor its lifestyle, not flight efficiency. It flies fast with rapid wingbeats, and its landings are about as graceful as a dropped rock. It simply lowers its trajectory, hitting the water fast from a low angle.
Landing is difficult for albatrosses because their lifestyle requires them to spend so much time in the air. Their wings are highly efficient, long and narrow. They will not slow the birds down as much as broader wings, so albatross landing speeds are often fast.
High speeds work for water landings but are usually too much for controlled landings on islands. After a few running steps, the birds typically fall forward awkwardly and slide headfirst on their chest. On account of such clumsiness, albatrosses are often called “gooney birds” or “mollymauks,” which is Dutch for “foolish gull.”
For most birds, the apparent simplicity of landing hides a complex, well-coordinated series of movements that offset the natural forces for flight. The intricate aspects of landing contribute to the amazing ability of birds to be masters of the air.
This article from Eldon Greij’s column “Amazing Birds” appeared in the January/February 2015 issue of BirdWatching
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