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.

Metabolic power input (P_chem) was recorded in 8 Pipistrellus nathusii flying in a wind tunnel at various speeds. We using the ¹³C-labelled sodium bicarbonate meathod to record CO2 production as a proxy for metabolic rate in flight. We converted CO2 production to watts (W) using a conversion factor of 20.1 W per ml CO2.  Mechanical power output (P_mech) was calculated using tomographical particle image velocimetry (tomoPIV), also presented as W.