Birds that use their wings for ‘flight’ in both air and water are expected to fly poorly in each fluid relative to single-fluid specialists; that is, these jacks-of-all-trades should be the masters of none. Alcids exhibit exceptional dive performance while retaining aerial flight. We hypothesized that alcids maintain efficient Strouhal numbers and stroke velocities across air and water, allowing them to mitigate the costs of their ‘fluid generalism’. We show that alcids cruise at Strouhal numbers between 0.10 and 0.40 – on par with single-fluid specialists – in both air and water but flap their wings ~ 50% slower in water. Thus, these species either contract their muscles at inefficient velocities or maintain a two-geared muscle system, highlighting a clear cost to using the same morphology for locomotion in two fluids. Additionally, alcids varied stroke-plane angle between air and water and chord angle during aquatic flight, expanding their performance envelope.

中文翻译：

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Alcids 在空气和水中以有效的 Strouhal 数“飞行”，但改变行程速度和角度

与单流体专家相比，使用翅膀在空气和水中“飞行”的鸟类在每种流体中的飞行性能都很差；也就是说，这些多面手应该是无人能及的。Alcids 表现出卓越的潜水性能，同时保持空中飞行。我们假设 alcids 在空气和水中保持有效的 Strouhal 数和行程速度，使它们能够减轻其“流体泛化”的成本。我们表明，alcids 在空气和水中以 0.10 到 0.40 之间的 Strouhal 数巡航——与单流体专家相当——但在水中拍动翅膀的速度要慢 50%。因此，这些物种要么以低效的速度收缩肌肉，要么保持两轮肌肉系统，这凸显了在两种流体中使用相同形态运动的明显成本。此外，