Data from: Mechanical power curve measured in the wake of pied flycatchers indicate modulation of parasite power across flight speeds

How aerodynamic power required for animal flight varies with flight speed determines optimal speeds during foraging and migratory flight. Despite its relevance, aerodynamic power provide an elusive quantity to measure directly in animal flight. Here we determine the aerodynamic power from wake velocity fields, measured using tomographical particle image velocimetry, of pied flycatchers flying freely in a wind tunnel. We find a shallow U-shaped power curve, which is flatter than expected by theory. Based on how the birds vary body angle with speed, we speculate that the shallow curve result from increased body drag coefficient and body frontal area at lower flight speeds. Including modulation of body drag in the model results in a more reasonable fit with data than the traditional model. From the wake structure we also find a single starting vortex generated from the two wings during the downstroke across flight speeds (1-9 m/s). This is accomplished by the arm wings interacting at the beginning of the downstroke, generating a unified starting vortex above the body of the bird. We interpret this as a mechanism resulting in uniform downwash and low induced power, which can help explain the higher aerodynamic performance in birds compared to bats.

动物飞行所需的气动功率（aerodynamic power）随飞行速度的变化规律，决定了其在觅食与迁徙飞行过程中的最优飞行速度。尽管该参数具有重要的研究价值，但直接测量动物飞行中的气动功率仍是一项极具挑战的工作。本研究通过层析粒子图像测速术（tomographical particle image velocimetry）测量风洞内自由飞行的斑姬鹟的尾流速度场，进而计算得到其气动功率。我们观测到一条平缓的U型功率曲线，其平缓程度超出了理论预期。基于鸟类身体倾角随飞行速度的变化规律，我们推测，该平缓曲线的成因是：在较低飞行速度下，鸟类的身体阻力系数（body drag coefficient）与身体迎风面积（body frontal area）均有所增加。相较于传统模型，在模型中引入身体阻力的调制项后，其与实验数据的拟合度更为合理。通过分析尾流结构，我们还发现，在1~9 m/s的全飞行速度区间内，斑姬鹟在下拍（downstroke）过程中，双翼会产生单个起始涡（starting vortex）。这一现象源于下拍初期双翼的翼臂发生相互作用，进而在鸟类身体上方形成一个统一的起始涡。我们认为，这一机制能够产生均匀的下洗流（downwash）并降低诱导功率（induced power），这或许可以解释为何鸟类的气动性能要优于蝙蝠。