The allometry of bird flight performance

Avian flight performance decreases with body size in birds, but previous work has been unable to define the underlying mechanism. Wingbeat frequency is hypothesized to ultimately constrain flight performance via muscular mechanical power output because frequency decreases with body size. I measured maximal burst take-off and vertical accelerating flight in 32 species of songbirds (Passeriformes), including the entire range of body mass in this clade (5-900 g). Jump forces against the ground were recorded with a forceplate. High-speed digital video captured the movement of morphological landmarks in order to estimate aerodynamic power requirements and dynamic morphology in flight. Surgically implanted gauges recorded the components of muscle power (muscle length change, force production, frequency) in the four largest species (Common raven, American crow, Black-billed magpie, and Gray jay). Flight performance and total aerodynamic power scaled with negative allometry, but were significantly influenced by foraging ecology. Species that forage on the ground had relatively lower jump impulses, shorter wings, higher wingbeat frequencies, and higher power output than species that forage on elevated substrates. I also found two unexpected internal scaling patterns. Both proportional muscle length change (muscle strain) and average cross-sectional area specific force (muscle stress) increased with size. Longer wingbeat cycles may permit more complete muscle activation in larger birds, thereby partially compensating for the constraint imposed by wingbeat frequency. These data offer the strongest support and the only direct evidence for power-limited scaling of flight performance to date