Low conversion efficiency of flight power increases with speed in the migratory bat Nathusius’ pipistrelle

The efficiency with which flying animals convert metabolic fuel to
mechanical power dictates an individual’s flight behaviour and energy
requirements. Despite the significance of this parameter, we lack
empirical data on conversion efficiency for most species. Furthermore, the
assumption that efficiency is constant across flight speeds has yet to be
empirically confirmed, revealing a major outstanding question in animal
flight. We show, through direct measurements of metabolic and aerodynamic
power, that conversion efficiency in the migratory bat (Pipistrellus
nathusii) increases with flight speed between 7.0 and 10.4%. Our findings
suggest that peak conversion efficiency in this species occurs near
maximum range speed, where the cost of transport is minimized. A
meta-analysis of 16 bird and 8 bat species revealed a strong scaling
relationship between estimated conversion efficiency and body mass, with
no discernible differences between bats and birds. This has profound
consequences for modelling flight behaviour as estimates assuming 23%
efficiency underestimate flight costs for P. nathusii by almost 50% on
average (36-62%). Our findings suggest that conversion efficiency may vary
around an ecologically relevant optimum speed and provides a crucial
baseline for investigating how this may drive the variation in conversion
efficiency between species.