Two theories have recently dominated the debate: one postulates that flight evolved in tree-dwelling ancestors that used their forelimbs to help them glide, while the other considers ancestral birds to be terrestrial dinosaurs that developed powered flight from the ground up.

An article by Kenneth P. Dial and two co-authors in the May 2006 issue of BioScience summarizes experimental evidence indicating that ancestral protobirds incapable of flight could have used their protowings to improve hindlimb traction and thus better navigate steep slopes and obstructions. By using their protowings in this way, they would presumably have had an advantage when pursuing prey and escaping from predators.

Dial and colleagues performed experiments on several species of juvenile galliform (chicken-like) birds, concentrating on chukar partridges. Chukars can run 12 hours after hatching, but they cannot fly until they are about a week old. Even before they are able to fly, however, the birds flap their developing wings in a characteristic way while running, which improves their ability to climb steep slopes and even vertical surfaces. Dial and colleagues have named this form of locomotion "wing-assisted Iincline running" (WAIR). After they are able to fly, chukars often use WAIR in preference to flying to gain elevated terrain, and exhausted birds always resort to WAIR.

Dial and colleagues describe experiments showing that if the surface area of chukar wings is reduced by plucking or trimming the feathers, WAIR becomes less effective for climbing slopes. Dial and colleagues propose that incipiently feathered forelimbs of bipedal protobirds may have provided the same advantages for incline running as have now been demonstrated in living juvenile birds. Their work thus supports a new theory about the evolution of flight in birds. WAIR, which the authors believe to be widespread in birds, appears to offer an answer to the question first posed by St. George Jackson Mivart in 1871: "What use is half a wing?"

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