Application of helicopter aerodynamics to hovering hummingbird flight shows that the power output required for hovering is directly and linearly dependent on wing disc loading. Wing disc loading takes into account both body weight and wing disc area and is the most realistic basis for evaluating the power output required for hovering. The similarity of data available for power input (energy metabolism during hovering) is probably due to similar wing disc loadings, even though species have different body weights. If all hummingbirds have similar efficiencies while hovering, we predict that species with different wing disc loadings should have different weight-specific metabolic costs for hovering. If species with different wing disc loadings are found to have similar metabolic costs for hovering, then efficiencies must vary.
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