Morphological Variations of Leading-Edge Serrations in Owls (Strigiformes)

Abstract

BackgroundOwls have developed serrations, comb-like structures, along the leading edge of their wings. Serrations were investigated from a morphological and a mechanical point of view, but were not yet quantitatively compared for different species. Such a comparative investigation of serrations from species of different sizes and activity patterns may provide new information about the function of the serrations.ResultsSerrations on complete wings and on tenth primary remiges of seven owl species were investigated. Small, middle-sized, and large owl species were investigated as well as species being more active during the day and owls being more active during the night. Serrations occurred at the outer parts of the wings, predominantly at tenth primary remiges, but also on further wing feathers in most species. Serration tips were oriented away from the feather rachis so that they faced into the air stream during flight. The serrations of nocturnal owl species were higher developed as demonstrated by a larger inclination angle (the angle between the base of the barb and the rachis), a larger tip displacement angle (the angle between the tip of the serration and the base of the serration) and a longer length. Putting the measured data into a clustering algorithm yielded dendrograms that suggested a strong influence of activity pattern, but only a weak influence of size on the development of the serrations.ConclusionsSerrations are supposed to be involved in noise reduction during flight and also depend on the aerodynamic properties that in turn depend on body size. Since especially nocturnal owls have to rely on hearing during prey capture, the more pronounced serrations of nocturnal species lend further support to the notion that serrations have an important function in noise reduction. The differences in shape of the serrations investigated indicate that a silent flight requires well-developed serrations.

The Hydrodynamic Noise Suppression of a Scaled Submarine Model by Leading-Edge Serrations

“…The notion that serrations have an important function in noise reduction was confirmed by Matthias Weger when he investigated the serrations on complete wings and on the 10th primary remiges of seven owl species [1]. Since the unsteady blade forces are of lower magnitude for a fan with sinusoidal leading edges than for a fan with straight leading edges [2], and torque fluctuations are significantly suppressed due to the passive flow control strategies implemented in the optimized vertical axis wind turbine model [3], even the effect of the stagnation point on the flow in front of the leading edge is observed to reduce drastically in the case of serrations [4].…”

Evolution and Ecology of Silent Flight in Owls and Other Flying Vertebrates

“…In essence, the comb is a type of vane fringe that differs from other vane fringes in two ways: (1) it has sufficient stiffness to withstand substantial deformation in the presence of aerodynamic forces imposed on it and (2) the proximal barbules on the barbs forming the comb are greatly reduced compared with the distal barbule (see fig. 1 in Weger and Wagner 2016 ).…”

“…The leading-edge comb (or serrations) is a row of curved barb tips ( Feo et al. 2016 ) that extend forward from the leading-edge of wing feather P10 and sometimes the leading-edge of the tip of P9, the alula (thumb feathers), and rarely, P8 and P7 ( Weger and Wagner 2016 ; Fig. 4A ).…”

“…In many owl species the comb also curves and twists upward, but there is substantial variation among species in comb morphology with respect to length, inclination angle, and tip displacement angle ( Weger and Wagner 2016 ). Within an individual, comb morphology also varies down the length of the wing.…”

“…Within an individual, comb morphology also varies down the length of the wing. Sick (1937) grouped the leading-edge comb into two categories, the longer, well-defined “Bubo type” with a tapered end and upward curve and the shorter, blunt-tipped “Surina type” (cited in Weger and Wagner 2016 ). Narrowly defining the comb excludes the shorter morphologies of many species of owl including members of the genera Athene , Bubo , Glaucidium , Ninox , Otus , Surina , and Tyto .…”