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Dial says that the relatively fixed angle of wing stroke among chukars has been confirmed by his lab in video recordings of more than 20 other bird species. He argued that it is a basic feature shared by all flying birds, and there are critical changes in fossilized shoulder bones, among late theropod dinosaurs and early avians, to suggest how this change came about. "The fundamental wing stroke we describe is plesiomorphic" -- meaning, an ancestral characteristic -- "and elementary to understanding critical elements of avian locomotion and perhaps its evolution," he said. The arc of wing stroke provides several advantages, he said. One is that it directs aerodynamic force at about 40 degrees above the horizontal, providing an airflow under the wings that boosts the bird's agility at lower energy cost. This would be very important for early birds with poorly developed feathers. And it also provides an air-brake for fledgling birds. If a young bird falls from a tree, for instance, it can flap its wings and this will slow its fall, thus preventing injury. Dial says that in the debate, both theories rest on the assumption that birds with proto-feathers could at first glide, at least a little way, until they eventually gained the ability to beat their wings for locomotion. But all the evidence among today's birds and on non-avian gliders suggests this: the gliding came later. And it came only after birds were able to flap their wings the right way, a process that was probably much simpler than thought.
Jennifer Viegas' blog: Born Animal |
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