A new idea about how birds became airborne is about to take wing. Studies of locomotion in partridges have now shown that these birds can move their wings in ways no one knew--back and forth as well as up and down. This newly discovered flapping helps these birds ascend inclines. If the slope is steep enough, they switch to moving their wings up and down, a progression that could reflect how avian ancestors evolved flight.
For almost a century, evolutionary biologists have argued about the origin of flight. Did avian ancestors evolve the ability to fly by first gliding from trees, or by getting a running start on flat ground as they held their feathered forelimbs stretched out? Neither scenario is quite right, says Kenneth Dial, a functional morphologist at the University of Montana, Missoula. His study suggests instead that the key innovation was the ability to move wings in different ways to scale steep inclines.
Dial's evidence comes from chukar partridges, which are relatives of chickens, quails, and turkeys. While helping Dial study flight dynamics in these birds, his 15-year-old son noticed that the chukars seemed to climb straight up bales of hay. Using high-speed video and devices that monitor acceleration, they found that newly hatched birds could walk up slopes of 45 degrees in a large part by flapping their wings at a different angle than when flying.
The net effect pushes the bird into the incline so that its feet don't slip--akin to spoilers on a race car. Adults could sprint up overhangs of 105 degrees, sometimes climbing 5 meters, switching to flying on the steeper slopes. It's possible, says Dial, that protobirds first moved like the chicks. Then because it helped them escape predators, they evolved a new way of moving their feathered forearms and thus equipped, they could flap away into the wild blue yonder.
The videos of the birds scampering up steep inclines are remarkable and have "blown the field wide open," says Kevin Padian, an evolutionary biologist at the University of California, Berkeley. But even Dial agrees that the flap over flight--now with three competing theories--will likely continue.
Dial's Science paper (with links to movies) 
Overview of Dial's research program 
General discussion on the evolution of flight in birds 
Discussion of dinosaur-to-bird transition