In WAIR, the wings and flight feathers are arranged and moved to generate net downward force over a significant part of the wing cycle, increasing traction for upslope running. In flight, of course, the flight stroke is operated to maximise lift, and thrust on take-off.
WAIR involves sophisticated modification of the basic rapid wing cycle that all flying birds have today. Rapid wing flapping is accomplished by strong pectoral muscles. The upstroke, dominantly a recovery stroke, is operated by the pectoral m. supracoracoideus muscle by way of a strong tendon that passes through the shoulder joint to insert on the upper side of the humerus. Rapid wing strokes are aided by a flexible furcula, and they are fundamental to take-off and landing in flying birds.
These systems leave visible traces on the skeleton. Thus we can say with confidence that Archaeopteryx did not have them, so did not have WAIR. Since Archaeopteryx is the most basal bird (1), WAIR is not relevant to the origin of flight. WAIR remains a fascinating later adaptation for ground running, but could not have evolved before the flight stroke was perfected.
I naturally prefer my own hypothesis for the origin of feathers and flight in birds, formulated jointly with Jere H. Lipps. This hypothesis involves flight "training" at zero ground speed and zero altitude, by way of display and fighting (2).
Department of Geology, University of California, Davis, California 95616
1. For example, K. Padian and L. M. Chiappe, Biol. Rev. 73, 1 (1998)
2. R. Cowen and J. H. Lipps. Proc. 3rd N. Am. Paleontol. Convention, vol. 1, 109 (1982); updated in 1990, 1995 and 2000 in successive editions of R. Cowen, History of Life (Blackwell). Text of all versions