Extreme lightweight structures: avian feathers and bones
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.