Feathers are the defining attribute of birds, and Archaeopteryx, widely accepted as the first known bird, was so identified by its feathers. Careful studies have determined that the wing feathers of Archaeopteryx are capable of generating lift, suggesting that it was capable of gliding, but no one has demonstrated that Archaeopteryx was capable of powered flight.
It is now recognized that birds evolved from dinosaurs, specifically Theropod (meat-eating) dinosaurs. I have long contended that all dinosaurs had feathers, and that the larger dinosaurs had feathers just as a rhinoceros has hair, i.e. not much and only in protected places. It is now widely accepted that most, if not all, theropod dinosaurs had feathers, and that dinosaurs were warm blooded. I maintain that feathers evolved as insulation along with the development of warm-bloodedness, and first appeared concurrent with bipedal locomotion in the Early Triassic ancestors of dinosaurs.
The inspiration for my origin of flight theory comes from watching a cock fight. (No, I have never been to a cock fight, but Hollywood studios have archival film footage of cock fights in less civilized countries, and I have seen films of them.) Cocks fight with their talons, which means that they have to leap into the air to unlimber their weapons. To get better elevation, they flap down with their wings to increase lift, simultaneously rotating their bodies back and thrusting their talons forward.
Deinonychus is a medium sized theropod dinosaur from the early Cretaceous that is thought to be very closely related to bird ancestors such as Archaeopteryx. It has an enlarged, sicle-like claw on its hind feet that is thought to have been used to disembowel its prey. (Deinonychus means "terrible claw".) If it had aerodynamic "wing" feathers on its forearms that were capable of generating lift when it beat its arms downward, it would make kicking upward into the belly of its prey much more effective. There are other raptors and early bird-like species that have a similar claw. It seems to me that having "wing" feathers that were capable of generating lift to free up their talons would have been a benefit to any number of small dinosaurs and bird ancestors, either for catching prey or for intraspecies conflict. This behavior would drive natural selection towards more powerful breast muscles and a stronger bone structure to support them, precisely the developments required for powered flight.
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