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

Albeit costly, flight allows birds to travel great distances in short time, making it a highly effective mode of locomotion, especially during migration. Understanding how birds use energy during flight is essential for studying their flight ecology. To fly, birds flap their wings accelerating surrounding air and generating the flight forces, where the rate of energy added to the wake represents flight mechanical power (Pmech). For flapping, birds utilize chemical energy in their flight muscles, which along with the metabolism of other body functions, constitutes the flight metabolic power (Pmet). The ratio between Pmech and Pmet is the energy conversion efficiency (ɳ), 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 modelling. Here we estimated, in vivo, ɳ from direct measurements of Pmet and Pmech using the 13C-labelled 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 (15.3% at 7.4 m s-1 for weight-specific power) occurring at ecologically relevant flight speeds. Our findings suggest tuning of performance to speeds most relevant for efficient transportation, with implications for modelling flight power based on mechanical models, as ɳ, a fundamental attribute in bird flight energetics, varies across flight speeds. We provide the data sets used for metabolic and mechanical power measurements, from which energy conversion efficiency was calculated. Also we provide a figure compiling all the NaBi experiments included in the analysis as well as the calibration experiment for the NaBi method.尽管飞行需付出高昂代价，但鸟类可借此在短时间内长距离移动，使其成为极为高效的运动方式，尤其在迁徙过程中。解析鸟类飞行过程中的能量利用机制，对于研究其飞行生态学具有重要意义。鸟类飞行时需振翅加速周围空气以产生飞行所需的气动力，此时尾流获得的能量速率即为飞行机械功率（Pmech）。振翅过程中，鸟类会消耗飞行肌肉储存的化学能，结合机体其他生理机能的代谢过程，共同构成飞行代谢功率（Pmet）。Pmech与Pmet的比值即为能量转换效率（ɳ），其大小取决于肌肉将燃料转化为机械功的能力（剩余能量以热能形式耗散），以及气动力产生过程中的能量损失。由于此前缺乏直接测量手段，相关研究要么假设η在所有飞行速度下均为恒定值（23%），要么依赖模型进行估算。本研究通过风洞实验，分别利用13C标记碳酸氢钠法（13C-labelled sodium bicarbonate method）与粒子图像测速法（particle image velocimetry, PIV），直接测量鸫夜莺的Pmet与Pmech，进而在活体状态下估算得到能量转换效率η。研究结果显示，η随飞行速度呈凹函数变化，在符合生态学意义的飞行速度区间内达到峰值：当以单位体重功率计算时，η的最大值为15.3%，对应飞行速度为7.4 m·s⁻¹。本研究结果表明，鸟类的飞行性能会适配于其高效迁移所需的最优速度；这一发现对于基于机械模型构建飞行功率模型具有重要启示——因为η作为鸟类飞行能量学的核心参数，其数值会随飞行速度发生变化。 本研究提供了用于代谢功率与机械功率测量的数据集，基于该数据集可计算得到能量转换效率。此外，本研究还附带一张汇总图，涵盖了分析中纳入的所有碳酸氢钠（NaBi）实验，以及碳酸氢钠方法的校准实验。