Thomas Bachmann

Morphometric characterisation of wing feathers of the barn owl Tyto alba pratincola and the pigeon Columba livia

BackgroundOwls are known for their silent flight. Even though there is some information available on the mechanisms that lead to a reduction of noise emission, neither the morphological basis, nor the biological mechanisms of the owls silent flight are known. Therefore, we have initiated a systematic analysis of wing morphology in both a specialist, the barn owl, and a generalist, the pigeon. This report presents a comparison between the feathers of the barn owl and the pigeon and emphasise the specific characteristics of the owls feathers on macroscopic and microscopic level. An understanding of the features and mechanisms underlying this silent flight might eventually be employed for aerodynamic purposes and lead to a new wing design in modern aircrafts.ResultsA variety of different feathers (six remiges and six coverts), taken from several specimen in either species, were investigated. Quantitative analysis of digital images and scanning electron microscopy were used for a morphometric characterisation. Although both species have comparable body weights, barn owl feathers were in general larger than pigeon feathers. For both species, the depth and the area of the outer vanes of the remiges were typically smaller than those of the inner vanes. This difference was more pronounced in the barn owl than in the pigeon. Owl feathers also had lesser radiates, longer pennula, and were more translucent than pigeon feathers. The two species achieved smooth edges and regular surfaces of the vanes by different construction principles: while the angles of attachment to the rachis and the length of the barbs was nearly constant for the barn owl, these parameters varied in the pigeon. We also present a quantitative description of several characteristic features of barn owl feathers, e.g., the serrations at the leading edge of the wing, the fringes at the edges of each feather, and the velvet-like dorsal surface.ConclusionThe quantitative description of the feathers and the specific structures of owl feathers can be used as a model for the construction of a biomimetic airplane wing or, in general, as a source for noise-reducing applications on any surfaces subjected to flow fields.

Flexural stiffness of feather shafts: geometry rules over material properties

SUMMARY Flight feathers of birds interact with the flow field during flight. They bend and twist under aerodynamic loads. Two parameters are mainly responsible for flexibility in feathers: the elastic modulus (Youngs modulus, E) of the material (keratin) and the geometry of the rachises, more precisely the second moment of area (I). Two independent methods were employed to determine Youngs modulus of feather rachis keratin. Moreover, the second moment of area and the bending stiffness of feather shafts from fifth primaries of barn owls (Tyto alba) and pigeons (Columba livia) were calculated. These species of birds are of comparable body mass but differ in wing size and flight style. Whether their feather material (keratin) underwent an adaptation in stiffness was previously unknown. This study shows that no significant variation in Youngs modulus between the two species exists. However, differences in Youngs modulus between proximal and distal feather regions were found in both species. Cross-sections of pigeon rachises were particularly well developed and rich in structural elements, exemplified by dorsal ridges and a well-pronounced transversal septum. In contrast, cross-sections of barn owl rachises were less profiled but had a higher second moment of area. Consequently, the calculated bending stiffness (EI) was higher in barn owls as well. The results show that flexural stiffness is predominantly influenced by the geometry of the feathers rather than by local material properties.

The three-dimensional shape of serrations at barn owl wings: towards a typical natural serration as a role model for biomimetic applications

Barn owl feathers at the leading edge of the wing are equipped with comb‐like structures termed serrations on their outer vanes. Each serration is formed by one barb ending that separates and bends upwards. This structure is considered to play a role in air‐flow control and noise reduction during flight. Hence, it has considerable potential for engineering applications, particularly in the aviation industry. Several publications have reported possible functions of serrations at artificial airfoils. However, only crude approximations of natural serrations have so far been investigated. We refer to these attempts as zero‐order approximations of serrations. It was the goal of this study to present a quantitative three‐dimensional characterization of natural serrations as first‐order approximations (mean values) and second‐order approximations (listed differences depending on the position of the serration along the leading edge). Confocal laser scanning microscopy was used for a three‐dimensional reconstruction and investigation with high spatial resolution. Each serration was defined by its length, profile geometry and curvature. Furthermore, the orientation of the serrations at the leading edge was characterized by the inclination angle, the tilt angle and the separation distance of neighboring serrations. These data are discussed with respect to possible applications of serration‐like structures for noise suppression and air‐flow control.

Barn Owl Flight

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.

Inner vane fringes of barn owl feathers reconsidered: morphometric data and functional aspects

It is a challenge to understand how barn owls (Tyto alba) reduce noise during flight to be able to hunt small mammals by audition. Several specializations of the wing and the wing feathers have been implicated in noise reduction. What has been overlooked so far are the fringes at the inner vanes of remiges. We demonstrated, by using precise imaging techniques combined with morphometric measurements and air‐flow studies, that these fringes 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 reduces sharp and noisy edges. This finding sheds new light on the mechanisms underlying noise reduction of flying owls.

The barn owl wing: an inspiration for silent flight in the aviation industry?

Barn owls are specialists in prey detection using acoustic information. The flight apparatus of this bird of prey is most efficiently adapted to the hunting behavior by reducing flight noise. An understanding of the underlying mechanisms owls make use of could help minimize the noise disturbances in airport or wind power plant neighborhood. Here, we characterize wings of barn owls in terms of an airfoil as a role model for studying silent flight. This characterization includes surface and edge specialization (serrations, fringes) evolved by the owl. Furthermore, we point towards possible adaptations of either noise suppression or air flow control that might be an inspiration for the construction of modern aircraft. Three-dimensional imaging techniques such as surface digitizing, computed tomography and confocal laser scanning microscopy were used to investigate the wings and feathers in high spatial resolution. We show that wings of barn owls are huge in relation to their body mass resulting in a very low wing loading which in turn enables a slow flight and an increased maneuverability. Profiles of the wing are highly cambered and anteriorly thickened, especially at the proximal wing, leading to high lift production during flight. However, wind tunnel experiments showed that the air flow tends to separate at such wing configurations, especially at low-speed flight. Barn owls compensated this problem by evolving surface and edge modifications that stabilize the air flow. A quantitative three-dimensionally characterization of some of these structures is presented.