Hummingbirds exhibit numerous specializations for high metabolism and high wingbeat frequencies such as modifications to the muscular and skeletal systems we expect the wing shape to be no exception. This activity requires among the highest mass-specific metabolic input and mechanical power output of any form of locomotion. Hovering hummingbirds exhibit the highest contractile frequencies of any vertebrate limb muscle, surpassed only by some sonic muscles and insect flight muscles. A side-by-side aerodynamic performance comparison of hummingbird wings and an advanced micro helicopter rotor shows that they are remarkably similar.
Quantitative flow visualization demonstrates that variation in hover power among hummingbird wings is driven by similar stable leading edge vortices that delay stall during the down- and upstroke. During the downstroke, the power required to hover decreases for larger AR wings at the angles of attack at which hummingbirds flap their wings ( p < 0.05). Our comparative analysis shows that AR has a modest influence on lift and drag forces, as reported earlier, but interspecific differences in power factor are large. The power (aerodynamic torque × angular velocity) required to lift weight depends on aerodynamic efficacy, which is measured by the power factor. We spun the wings at angular velocities and angles of attack that are representative for every species and measured lift and torque more precisely. To test this more carefully, we compare the quasi-steady hover performance of 26 wings, from 12 hummingbird taxa. Previous quasi-steady experiments with a wing spinner set-up provide no support for this prediction.
Based on helicopter and aeroplane design theory, we expect that hummingbird wing aspect ratio (AR), which ranges from about 3.0 to 4.5, determines aerodynamic efficacy. This unique flight behaviour comes, however, at high energetic cost. Hummingbirds are the only birds that can sustain hovering.
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