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

Abstract

Background: Owls 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 owl's 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 owl's 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.

Results: A 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.

Conclusion: The 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.

Figure 1

Feather position and measured feather parameters. (A) Position of the investigated feathers in the barn owl (left) and the pigeon (right); scale bar: 10 cm. (B) Investigated parameters on the flight feather (p5 of a barn owl). Measurements were taken every 10% of feather length. (C) Scanning electron microscopy pictures of two connected barbs at 60% of the inner vane of feather p10 of the barn owl (above) and the pigeon (below) from dorsal (left) and ventral (right) view (bs: barb shaft, hr: hook radiate, br: bow radiate, p: pennulum, h: hooklet); scale bar 200 μm. (D) Investigated barb parameters.

Figure 2

Depth of vane in barn owl and pigeon wing feathers. Normalised depth of the outer and inner vane of remiges (A, B) and coverts (C, D) in the barn owl (A, C) and the pigeon (B, D). The colours indicate different feathers. Their position is presented at the wing. The depth was measured at right angles to the rachis and was then normalised with respect to the whole length of vane.

Figure 3

Asymmetry of barn owl and pigeon wing feathers. Asymmetry index of depth. Presents the asymmetry of the depth in the remiges (A, B) and coverts (C, D) in the barn owl (A, C) and the pigeon (B, D). The colours indicate different feathers. Their position is presented at the wing.

Figure 4

Length of barbs in barn owl and pigeon wing feathers. The normalised length of barbs of the inner (iv) and outer vane (ov) of remiges (A, B) and coverts (C, D) from the barn owl (A, C) and the pigeon (B, D) are shown. The length was measured from the base to the tip and then normalised with respect to the whole length of vane. The area outside the dotted lines indicates regions of unconnected barbs forming the plumulaceous barbs (in both species), the fringes (in the barn owl) or serrations (p10 and gpc10 in the barn owl).

Figure 5

Angle between barbs and rachis of wing feathers of the barn owl and the pigeon. Demonstrates the angles between the barbs and the rachis at the inner (iv) and outer vane (ov) of remiges (A, B) and coverts (C, D) from the barn owl (A, C) and the pigeon (B, D). The colours indicate different feathers. Their position is presented at the wing.

Figure 6

Barb density of wing feathers of the barn owl and the pigeon. The number of barbs per cm at the inner (iv) and outer vane (ov) of remiges (A, B) and coverts (C, D) from the barn owl (A, C) and the pigeon (C, D) are shown. The mean barb density per cm was calculated by dividing the total number of barbs by 10% (respectively 20%) of vane's length. The colours indicate different feathers. Their position is presented at the wing.

Figure 7

Details of a feather. (A-E) Details of the barn owl's feather p10. (A) Serrations at the outer vane's leading edge. (B) Fringes at the inner vane's trailing edge. (C) Velvet-like dorsal surface of the inner vane. (F-J) Details of the pigeon's feather p10. (F) Leading edge of the outer vane. (G) Trailing edge of the inner vane. (H) Dorsal surface of the inner vane; scale bar: 1 mm. (D, I) Qualitative illustration of the porosity (translucency) of black dyed inner vanes of feather gsc5 of the barn owl (D) and the pigeon (I). (E, J) Plumulaceous barbs of feather gsc5 of the barn owl (E) and the pigeon (J); scale bar: A-D and F-I: 1 mm, E and J: 5 mm.

Figure 8

Length of radiates of the outer vane of three different wing feathers from the barn owl and the pigeon. The mean length of the radiates of the outer vanes of the feathers p10, s8 and gpc1 in the barn owl (grey) and the pigeon (white) is depicted. Each diagram is divided into hook radiates (hr-left) and bow radiates (br-right). Additionally, each radiate was divided into radiate base (r) and pennulum (p). SEM: standard error of the mean.

Figure 9

Length of radiates of the inner vane of three different wing feathers from the barn owl and the pigeon. The mean length of the radiates of the inner vanes of the feathers p10, s8 and gpc1 in the barn owl (grey) and the pigeon (white). Each diagram is divided into hook radiates (left) and bow radiates (right). Additionally, each radiate can be divided into radiate base and pennulum. SEM: standard error of the mean.