Nature-Inspired Fluid Mechanics

Owls ( Strigiformes ) are nocturnal birds of prey that are known for their silent flight. For a long time, the underlying mechanisms were not well understood. In a comprehensive study, we have characterized the flight apparatus of one representative of owls, the barn owl ( Tyto alba pratincola ), to advance beyond the phenomenological description provided so far. The barn owl wing is adapted to slow flight as indicated by a low wing loading, an elliptical shape, a high camber and a specific thickness distribution. Further, feather specializations can be found: 1.) serrations at the leading edge of the wing, 2.) a velvety dorsal surface texture, and 3.) fringes at the inner vanes of remiges. Quantitative characterizations of these structures revealed that serrations had a uniform shape, but the length depended on their position on the wing. The velvety dorsal surface texture differed between the inner and outer vanes which is a consequence of different functions (air flow control, friction reduction). The fringes were observed to merge into neighboring feather vanes by gliding into grooves at the lower wing surface to create a smooth airfoil. Besides anatomical data, material properties and wearing effects of feather keratin of rachises and barbs were obtained.