Flight is not the exclusive domain of birds; mammals (bats), insects, and some fish have independently developed this ability by the process of convergent evolution. Birds, however, greatly outperform other flying animals in efficiency and duration; for example the common swift (Apus apus) has recently been reported to regularly fly for periods of 10 months during migration. Birds owe this extraordinary capability to feathers and bones, which are extreme lightweight biological materials. They achieve this crucial function through their efficient design spanning multiple length scales. Both feathers and bones have unusual combinations of structural features organized hierarchically from nano- to macroscale and enable a balance between lightweight and bending/torsional stiffness and strength. The complementary features between the avian bone and feather are reviewed here, for the first time, and provide insights into nature's approach at creating structures optimized for flight. We reveal a novel aspect of the feather vane, showing that its barbule spacing is consistently within the range 8–16 μm for birds of hugely different masses such as Anna's Hummingbird (Calypte anna) (4 g) and the Andean Condor (Vultur gryphus) (11,000 g). Features of the feather and bone are examined using the structure-property relationships that define Materials Science. We elucidate the role of aerodynamic loading on observed reinforced macrostructural features and efficiently tailored shapes adapted for specialized applications, as well as composite material utilization. These unique features will inspire synthetic structures with maximized performance/weight for potential use in future transportation systems.

中文翻译：

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极轻的结构：鸟类羽毛和骨骼

飞行并不是鸟类的专属领域；哺乳动物（蝙蝠）、昆虫和一些鱼类通过趋同进化的过程独立发展了这种能力。然而，鸟类在效率和持续时间上大大优于其他飞行动物。例如，最近有报道称普通雨燕 (Apus apus) 在迁徙期间定期飞行 10 个月。鸟类将这种非凡的能力归功于羽毛和骨骼，它们是极轻的生物材料。他们通过跨越多个长度尺度的高效设计实现了这一关键功能。羽毛和骨骼都具有不同寻常的结构特征组合，从纳米级到宏观级分层组织，并能够在轻量级和弯曲/扭转刚度和强度之间取得平衡。此处首次回顾了鸟类骨骼和羽毛之间的互补特征，并提供了对自然创造飞行优化结构的方法的见解。我们揭示了羽毛叶片的一个新方面，表明它的羽叶间距始终在 8-16 微米范围内，对于质量差异很大的鸟类，如安娜蜂鸟 (Calypte anna) (4 g) 和安第斯秃鹰 (Vultur gryphus) （11,000 克）。使用定义材料科学的结构-特性关系检查羽毛和骨骼的特征。我们阐明了空气动力学载荷对观察到的增强宏观结构特征和适用于专门应用的有效定制形状以及复合材料利用的作用。