A Nova episode currently in reruns on PBS features bird fossils discovered in China that have shed new light on the origin of flight in birds. Wind tunnel tests on a life-like model suggest a traditional assumption about hip articulation in fossil birds is wrong.
Instead of attaching in a running posture capable of holding the animal's weight, the hind legs attach at a different angle. Researchers call it "splayed" posture, and it is found in crocodiles and other branches of reptile phylogeny. Splayed articulation allows the legs to extend backwards toward the tail, useful for actions like walking on four legs, diving, leaping and gliding.
Running on splayed legs means less power and speed, so the new fossils make the hypothesis that birds evolved flight by being able to run at flight speed and leap into a glide less likely. Part of that hypothesis is the idea that the large middle claw on the hind legs was a "killing claw" thrust at victims using the same power and articulation needed for running. Splay posture suggests other functional possibilities, and the claw may instead have aided in climbing, perching and propelling the protobird from a branch.
Instead of jumping from the ground into flight, the first birds dove into glides from a perch.
There is one more assumption that must be questioned, that protobirds dove into the air. More likely, they dove into water, a more buoyant fluid that creates lift at lower speeds and weaker wing strokes. The fossil record of birds is consistent with a marine origin, with diving birds like loons resembling fossil ancestors more than other modern lineages.
Swimming is easier than flying, and wings likely evolved as swimming appendages before developing the power for airborne flight.