Effects of wingbeat kinematics and muscle size on maximum flight speed in hummingbirds

Flight speed has important fitness consequences on flying animals, but how the parts of the flight apparatus limit top speed remains unknown. Morphology, kinematics, and flight performance are strongly correlated with species and sex class, which complicates attempts to dissect the contribution of each element to flight performance. We performed two experiments to assess how sexual dimorphism within a species and morphological variation between species influence maximum flight speed. Burst capacity is often hypothesized to be representative of generalized flight performance and may be correlated with top speed. In the first experiment, we found top speed and burst capacity were negatively correlated in female but not male Archilochus alexandri. In experiment two, we used path analysis to disentangle the relative contributions of muscle size and wingbeat kinematics on maximum flight speed in both sexes of four species: A. alexandri Calypte anna, C. costae, and Selasphorus sasin. Considered separately, muscle size was more strongly associated with variation in both flight speed and burst capacity compared with wingtip velocity. In a post-hoc model comparing muscle size and wingbeat kinematics, only muscle size remained a significant predictor for both assays, but substantial variance in top speed remains unexplained. There was no significant correlation between flight speed and burst capacity despite sex-specific differences, suggesting flight muscle composition may be an important predictor of performance for a given flight assay. Overall, top speed and burst capacity represent uncorrelated measures of flight 'performance,' and significant predictors of flight speed remain unexplored.