Transition from leg to wing forces during take-off in birds

Abstract

SummaryTake-off mechanics are fundamental to the ecology and evolution of flying animals. Recent research reveals that initial take-off velocity in birds is driven mostly by hindlimbs forces. However, the contribution of the wings during the transition to air is unknown. To investigate this transition, we integrated measures of both leg and wing forces during take-off and the first three wingbeats in zebra finch (Taeniopygia guttata, 15g, N=7) and diamond dove (Geopelia cuneata, 50g, N=3). We measured ground-reaction forces produced by the hindlimbs using a perch mounted on a force-plate, whole body and wing kinematics using high-speed video, and aerodynamic forces using particle image velocimetry (PIV). Take-off performance was generally similar between species. When birds were perched, an acceleration peak produced by the legs contributed to 85±1% of the whole body resultant acceleration in finch and 77±6% in dove. At lift-off, coincident with the start of the first downstroke, the percentage of hindlimb contribution to initial flight velocity was 93.6±0.6% in finch and 95.2±0.4% in dove. In finch, the first wingbeat produced 57.9±3.4% of the lift created during subsequent wingbeats compared to 62.5±2.2% in dove. Advance ratios were < 0.5 in both species, even when taking self-convection of shed vortices into account, so it was likely that wing-wake interactions dominated aerodynamics during wingbeats 2 and 3. These results underscore the relatively low contribution of the wings to initial take-off, and reveal a novel transitional role for the first wingbeat in terms of force production.