Morphological correlates of silent flight
The barn owl wing (left side) has a larger area than that of the similarly-sized pigeon (right side). This means that the wing load is barn owls is low, allowing slow, and, thus, silent flight. Several plumage specializations have been reported in the barn owl that influence the flow over the wing: the dorsal surfaces of the feathers have an increased roughness formed by elongations of hook radiates, called pennula. The 10th primary and 10th greater primary covert, which form the leading edge of the distal wing, are equipped with serrations at the outer vanes (see anterior border in figure). Furthermore, every single feather of the wing is surrounded by a fringed structure due to unconnected barb endings (see posterior border in figure). The three-dimensional structure of the serrations was quantified leading to a first-order model serration that might be used in devices operating a similar Reynolds numbers as the barn owl (Figure on serrations). We also showed that the fringes of the inner vane merge into neighboring feather vanes by gliding into the grooves at the lower wing surface that are formed by parallel oriented barb shafts. The connection of adjacent feathers results in a smooth lower wing surface and thus reducing sharp and noisy edges (Figure on feather structure). This finding sheds new light on the mechanisms underlying noise reduction of flying owls.
Bachmann T, Wagner H, Tropea C (2012) Inner vane fringes of barn owl feathers reconsidered: morphometric data and functional aspects. J Anatomy (doi: 10.1111/j.1469-7580.21012.01504.x).
Bachmann T, Emmerlich J, Baumgartner W, Schneider J, Wagner H (2012) Flexural stiffness of feather shafts: geometry rules over material properties. J Exp Biol 215: 405-415.
Bachmann T and Wagner H (2011) The three-dimensional shape of serrations at barn owl wings: towards a typical natural serration as a role model for biomimetic applications. J Anatomy (doi 10.1111/,.1469-7580.2011.01384.x)
Bachmann T, Klän S, Baumgartner W, Klaas M, Schröder W and Wagner H (2007): Morphometric characterisation of wing feathers of the barn owl (Tyto alba pratincola) and the pigeon (Columba livia). Frontiers in Zoology 4:23.