Energy conversion efficiency peaks at intermediate flight speed in a migratory songbird

Albeit costly, flight allows birds to travel great distances in a short time, making it a highly effective mode of locomotion, especially during migration.^1,2 Understanding how birds use energy during flight is essential for studying their flight ecology.³ To fly, birds flap their wings, accelerating surrounding air and generating flight forces, where the rate of energy added to the wake represents flight mechanical power (P_mech).⁴ For flapping, birds utilize chemical energy in their flight muscles, which, along with the metabolism of other body functions, constitutes the flight metabolic power (P_met).^1,5 The ratio between P_mech and P_met is the energy conversion efficiency (ɳ),^6,7 which depends on the muscle's ability to convert fuel into work (the rest being dissipated as heat) and on the energy losses during aerodynamic force production. Due to lack of direct measurements, ɳ has been assumed constant across speeds (23%) or relied upon for modeling.^4,7 Here, we estimated, in vivo, ɳ from direct measurements of P_met and P_mech using the ¹³C-labeled sodium bicarbonate method and particle image velocimetry, respectively, in thrush nightingales flown in a wind tunnel. We found that ɳ varied as a concave function with flight speed, with a maximum ɳ of 15.3% within the range of 7-8 m s^-1, occurring at ecologically relevant flight speeds. Our findings suggest tuning of performance to speeds most relevant for efficient transportation, with implications for modeling flight power,^4,8 as ɳ, a fundamental attribute in bird flight energetics, varies across flight speeds.