Mechanics of wing-assisted incline running (WAIR)

A recently discovered locomotor behavior, wing-assisted incline running (WAIR), allows fully volant animals to 'run' up vertical obstacles. Such a task would appear to be especially formidable for bipeds, yet WAIR is used preferentially by ground-dwelling birds, specifically chukar partridge Alectoris chukar, to reach refugia. The basic locomotor mechanics that enable this behavior are not fully understood. For instance, are there functional differences at the level of the wing during WAIR and free flight, and do the hindlimbs actively participate in propulsion during WAIR? To investigate wing function during these activities we used accelerometry to compare the instantaneous whole-body acceleration during WAIR and ascending free flights at a similar climb angle. Throughout a substantial portion of the wingbeat cycle, chukars engaged in WAIR experienced an acceleration oriented towards the substrate, whereas during ascending free flights the acceleration of the center of mass was parallel to the direction of travel. We investigated whether the animals were using their hindlimbs for propulsion, rather than for some other function (e.g. to maintain balance), by measuring ground reaction forces (GRF) during bouts of WAIR. Estimates of the contribution of the hindlimbs towards the vertical external work done by the bird were 98 +/- 8% of the total at an incline of 60 degrees (the steepest angle that birds were able to negotiate without the use of their forelimbs). During vertical (90 degrees ) bouts of WAIR the hindlimb contribution was 37 +/- 5% of the total external work. Yet, the magnitude of the peak GRF at 90 degrees was 175% of the value generated during level walking, revealing that birds engaged in WAIR do generate sizeable hindlimb forces even during vertical ascents. These data support the hypothesis that forelimbs are enabling hindlimb function, and we argue that this represents a locomotor strategy which may have been used by proto-birds during the evolution of flight.