Interspecific variation in the structural properties of flight feathers in birds indicates adaptation to flight requirements and habitat

Abstract

Summary 1.The functional significance of intra-and interspecific structural variations in the flight feathers of birds is poorly understood. Here, a phylogenetic comparative analysis of four structural features (rachis width, barb and barbule density and porosity) of proximal and distal primary feathers of 137 European bird species was conducted. 2. Flight type (flapping and soaring, flapping and gliding, continuous flapping or passerine type), habitat (terrestrial, riparian or aquatic), wing characteristics (wing area, S and aspect ratio, AR) and moult strategy were all found to affect feather structure to some extent. Species characterized by low wing-beat frequency flight (soaring and gliding) have broader feather rachises (shafts) and feather vanes with lower barb density than birds associated with more active flapping modes of flight. However, the effect of flying mode on rachis width disappeared after controlling for S and AR, suggesting that rachis width is primarily determined by wing morphology. 3. Rachis width and feather vane density are likely related to differences in force distribution across the wingspan during different flight modes. An increase in shaft diameter, barb density and porosity from the proximal to distal wing feathers was found and was highest in species with flapping flight indicating that aerodynamic forces are more biased towards the distal feathers in flapping flyers than in soarers and gliders. 4. Habitat affected barb and barbule density, which was greatest in aquatic species, and within this group, barb density was greater in divers than non-divers, suggesting that the need for water repellency and resistance to water penetration may influence feather structure. However, we found little support for the importance of porosity in water repellency and water penetration, because porosity was similar in aquatic, riparian and terrestrial species and among the aquatic birds (divers and non-divers). We also found that barb density was affected by moult pattern. 5. Our results have broad implications for the understanding of the selection pressures driving flight feather functional morphology. Specifically, the large sample size relative to any previous studies has emphasized that the morphology of flight feathers is the result of a suite of selection pressures. As well as routine flight needs, constraints during moulting, habitat (particularly aquatic) and migratory requirements also affect flight feather morphology. Identifying the exact nature of these trade-offs will perhaps inform the reconstruction of the flying modes of extinct birds.

A phylogenetic comparative analysis reveals correlations between body feather structure and habitat

“…Nevertheless, in highly specialized aquatic species it is often difficult to deduce the relative functions of different body feather structures; in terrestrial species that are much less exposed to water penetration, plumage appears adapted to repel, rather than to withstand, infiltration due to high water pressures encountered during diving (Stephenson & Andrews ; Rijke & Jesser ; Pap et al . ).…”

Adventitious feather replacement favours a more rapid regeneration of primaries over rectrices in two passerine bird species

“…) together with flight feathers, or by performing detailed comparative studies that include species with different life histories (Pap et al . ).…”

Rachis morphology cannot accurately predict the mechanical performance of primary feathers in extant (and therefore fossil) feathered flyers

“…The differing mechanical demands upon the feathers of these species are likely to be driving the differences in the observed relationships between structural properties and morphology. Indeed, flight style does appear to affect feather morphology [10]. Therefore, if we are to determine the flight capabilities of extinct birds from morphology-based, predicted structural properties, we must first have an idea of their flight styles, thus creating a circular argument.…”

“…Conversely, in blackcaps ( Sylvia atricapilla ), although Ø r and feather mass explain much of the variation in mechanical performance, migratory individuals have stiffer rachises than sedentary individuals when controlling for these parameters [9]. Therefore, although differing flight demands may drive structural and material differences in feathers [10], rachis properties may not solely be reflected in external morphological measures [11]. Current knowledge regarding the structural properties of feather rachises is primarily based on cantilever tests, in which the feather is fixed at the calamus and its deflection measured under loading [7,8,12].…”