What use is half a wing in the ecology and evolution of birds?

The use of incipient wings during ontogeny in living birds reveals not only the function of these developing forelimbs in growing birds' survival but also the possible employment of protowings during transitional stages in the evolution of flight. When startled, juvenile galliform birds attempt aerial flight even though their wings are not fully developed. They also flap their incipient wings when they run tip precipitous inclines, a behavior we have described as wing-assisted incline running (WAIR), and when they launch from elevated structures. The functional benefit of beating these protowings has only recently been evaluated. We report the first ontogenetic aerial flight performance for any bird using a ground bird, the chukar partridge (Alectoris chukar), as a model species. 1,We provide additional ontogenetic data on WAIR, a recently described locomotor mode in which fully or even partially developed flapping forelimbs, are recruited to increase hindlimb traction and escape performance. We argue that avian ancestors may have used WAIR as an evolutionary transition from bipedal locomotion to flapping flight.