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

Abstract

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.

Fig. 1

Representatives of the five different remiges investigated: (A) 10th primary; (B) 5th primary; (C) 1st primary; (D) 4th secondary; (E) 9th secondary. Note the basal downs (plumulaceous barbs) at the base of each feather. Here, barbs separate to create a fluffy structure, which is used among other things for thermal insulation. Scale bar: 5 cm.

Fig. 2

Morphometrical parameters measured for the characterization of fringes at barn owl feathers. The length of fringes (mm) and their density (fringes per mm) were measured every 10% of the inner vane.

Fig. 3

Schematic of the feather arrangement (sequential secondaries) for the high-speed video analysis. The feathers are positioned as in a spread wing of a barn owl. The artificial wing was subjected to a flow field of V_max = 5 m s⁻¹. The area of interest was chosen to cover a central part of overlapping vanes (square). The schematic is not to scale.

Fig. 4

Example inner vane fringes of one 10th primary of the barn owl in different magnification: (A, B) dorsal; (C) ventral. Note also the serrated leading edge of the outer vane and the velvety dorsal surface of the feather. (D) SEM picture of an example barb of the inner vane (ventral). (a′) Hook radiates (anterior barbules); (b′) barb shaft; (c′) bow radiates (posterior barbules); (d′) hooklets (hamuli); (e′) pennula. (E) SEM picture of an example fringe (ventral). (a′) Hook radiates; (b′) barb shaft; (c′) bow radiates. Note the absence of hooklets.

Fig. 5

Scheme of a ventral barn owl wing. The arrows indicate the orientation of fringes in principle.

Fig. 6

Selected images of a high-speed film of two adjacent secondaries (A: s7, s8) under static conditions (B) and subjected to a flow field (C) (ventral view). (B) Under static conditions, the fringes of s7 and the inner vane of s8 have a certain distance indicated by the shadow. The fringes are bent slightly but do not align with the adjacent barbs. (C) Both vanes approach when subjected to a flow field. Thereby, fringes merge tightly into the grooves formed by the adjacent barb shafts.

Fig. 7

Comparison of inner vanes of neighboring secondaries (s7 and s8) from lower side. (A) Barn owl (Tyto alba); (B) common buzzard (Buteo buteo); (C) Harris hawk (Parabuteo unicinctus); (D) pigeon (Columba livia); (E) mute swan (Cygnus olor). Scale bars: 1 mm.