Scaling of mechanical power output during burst escape flight in the Corvidae.

Avian locomotor burst performance (e.g. acceleration, maneuverability) decreases with increasing body size and has significant implications for the survivorship, ecology and evolution of birds. However, the underlying mechanism of this scaling relationship has been elusive. The most cited mechanistic hypothesis posits that wingbeat frequency alone limits maximal muscular mass-specific power output. Because wingbeat frequency decreases with body size, it may explain the often-observed negative scaling of flight performance. To test this hypothesis we recorded in vivo muscular mechanical power from work-loop mechanics using surgically implanted sonomicrometry (measuring muscle length change) and strain gauges (measuring muscle force) in four species of Corvidae performing burst take-off and vertical escape flight. The scale relationships derived for the four species suggest that maximum muscle-mass-specific power scales slightly negatively with pectoralis muscle mass (M(-0.18)(m), 95% CI: -0.42 to 0.05), but less than the scaling of wingbeat frequency (M(-0.29)(m), 95% CI: -0.37 to -0.23). Mean muscle stress was independent of muscle mass (M(-0.02)(m), 95% CI: -0.20 to 0.19), but total muscle strain (percent length change) scaled positively (M(0.12)(m), 95% CI: 0.05 to 0.18), which is consistent with previous results from ground birds (Order Galliformes). These empirical results lend minimal support to the power-limiting hypothesis, but also suggest that muscle function changes with size to partially compensate for detrimental effects of size on power output, even within closely related species. Nevertheless, additional data for other taxa are needed to substantiate these scaling patterns.