Aerodynamics of the hovering hummingbird

Despite profound musculoskeletal differences, hummingbirds (Trochilidae) are widely thought to employ aerodynamic mechanisms similar to those used by insects. The kinematic symmetry of the hummingbird upstroke and downstroke(1-3) has led to the assumption that these halves of the wingbeat cycle contribute equally to weight support during hovering, as exhibited by insects of similar size(4). This assumption has been applied, either explicitly or implicitly, in widely used aerodynamic models(1,5-7) and in a variety of empirical tests(8,9). Here we provide measurements of the wake of hovering rufous hummingbirds (Selasphorus rufus) obtained with digital particle image velocimetry that show force asymmetry: hummingbirds produce 75% of their weight support during the downstroke and only 25% during the upstroke. Some of this asymmetry is probably due to inversion of their cambered wings during upstroke. The wake of hummingbird wings also reveals evidence of leading-edge vortices created during the downstroke, indicating that they may operate at Reynolds numbers sufficiently low to exploit a key mechanism typical of insect hovering(10,11). Hummingbird hovering approaches that of insects, yet remains distinct because of effects resulting from an inherently dissimilar-avian-body plan.