|MadSci Network: Zoology|
First we need to define what is meant by flying up-side down. For argument's sake, lets say: A sustained powered flight in an inverted position. As far as I know there is no record of any bird capable of this. What prevents birds from being able to achieve sustained flight up-side down is physics. The bird is aerodynamically designed to fly right side up. There are several complex processes involved in attaining flight. A quick physics lesson: In order for a bird to fly it must obtain an upward force call "lift". This applies to airplanes as well. After all, a plane's wings are designed on the basic physics of the bird wing. The shape of a birdís wing enables the bird to achieve lift because the wing is basically an airfoil. The front edge and the upper surface is more convex than the lower surface. So, when air strikes the leading edge of the wing, it divides- some air passes underneath the wing and the rest passes across the upper surface. Since the upper surface of the wing is curved, it has a longer surface than the underside of the wing. In order for air to travel to the rear of the wing at more or less the same time as the air on the underside, the air on the upper surface must travel faster than the air underneath the wing. The faster air travels across a surface, the lower the pressure it exerts on that surface. As a result, the wingís upper surface experiences a lower pressure than under the surface and produces lift. There are other processes involved as well including thrust and drag. But to keep things simple we'll avoid those right now. If we understand the physics of an airfoil or wing, then we can see that it works in only one direction.. up. An inverted wing will no longer provide lift. This is why birds are not capable of sustained flight in an inverted position. To learn more about the physics of flight, follow this link. http://www.ucmp.berkeley.edu/vertebrates/flight/physics.html Okay I know what your thinking: if a plane is based on the physics of a bird, and planes can fly up-side down why can't a bird do the same? This is a zoology question right? My strength is nature not physics, so I will let those with greater interest post that question in the physics category for the full answer. But...I believe the problem with birds versus planes is the direction of thrust. In a plane, thrust is provided by the engines or propellors and this force is always parallel to the wing. In birds, the wings are additionally providing the thrust through flapping. This makes it harder for birds to compensate and remain inverted like planes can. Please don't quote me on this. Back to the original question. What birds can fly upside down? Although they can't "sustain" an inverted flight, many birds can turn over for brief periods while in the air. Birds are quite agile and maneuverable and the ability to turn up-side down could be beneficial in avoiding capture, hunting prey, or attracting the attention of a mate. For example, the courtship display flight of the Peregrine Falcon involves dives, barrel rolls and summersaults. Falcons have also been known to turn upside down briefly to grasp prey on the wing. Bald Eagles have a dramatic courtship display in which the male and female clasp talons together in flight- sometimes one bird appears to turn upside down to do so. In the "Audubon Nature Encyclopedia" there is a section on the aerobatics of birds. The book describes Osprey doing loops and summersaults in playful flight. The text goes on to relate a story of a Bald Eagle that was observed warding off intruders by turning up-side down and remaining in the inverted position for several wing beats without great loss of altitude. If you had the opportunity to see David Attenbouroughs latest nature series, "The Life of Birds", there is great footage of Sea Eagles battling for territory. In the scene, one bird turns completely up side down to grasp the talons of the intruding bird. This position is sustained for many seconds. The anecdotal info I found on the aerobatics of birds mostly covered fast agile species like falcons, eagles and hawks. I would not rule out the possibility of many other birds being capable of this feat, but it is not as easily documented in smaller species. I hope this helps to answer your question Steve A Williams firstname.lastname@example.org
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