Transition from wing to leg forces during landing in birds.

Abstract

Transitions to and from the air are critical for aerial locomotion and likely shaped the evolution of flying animals. Research on take-off demonstrates that legs generate greater body accelerations compared with wings, and thereby contribute more to initial flight velocity. Here, we explored coordination between wings and legs in two species with different wingbeat styles, and quantified force production of these modules during the final phase of landing. We used the same birds that we had previously studied during take-off: zebra finch (Taeniopygia guttata, N=4) and diamond dove (Geopelia cuneata, N=3). We measured kinematics using high-speed video, aerodynamics using particle image velocimetry, and ground-reaction forces using a perch mounted on a force plate. In contrast with the first three wingbeats of take-off, the final four wingbeats during landing featured ~2 times greater force production. Thus, wings contribute proportionally more to changes in velocity during the last phase of landing compared with the initial phase of take-off. The two species touched down at the same velocity (~1 m s(-1)), but they exhibited significant differences in the timing of their final wingbeat relative to touchdown. The ratio of average wing force to peak leg force was greater in diamond doves than in zebra finches. Peak ground reaction forces during landing were ~50% of those during take-off, consistent with the birds being motivated to control landing. Likewise, estimations of mechanical energy flux for both species indicate that wings produce 3-10 times more mechanical work within the final wingbeats of flight compared with the kinetic energy of the body absorbed by legs during ground contact.