Decreased hydrophobicity of iridescent feathers: a potential cost of shiny plumage

Abstract

SUMMARYHonest advertisement models posit that sexually selected traits are costly to produce, maintain or otherwise bear. Brightly coloured feathers are thought to be classic examples of these models, but evidence for a cost in feathers not coloured by carotenoid pigments is scarce. Unlike pigment-based colours, iridescent feather colours are produced by light scattering in modified feather barbules that are characteristically flattened and twisted towards the feather surface. These modifications increase light reflectance, but also expose more surface area for water adhesion, suggesting a potential trade-off between colour and hydrophobicity. Using light microscopy, spectrometry, contact angle goniometry and self-cleaning experiments, we show that iridescent feathers of mallards, Anas platyrhynchos, are less hydrophobic than adjacent non-iridescent feathers, and that this is primarily caused by differences in barbule microstructure. Furthermore, as a result of this decreased hydrophobicity, iridescent feathers are less efficient at self-cleaning than non-iridescent feathers. Together, these results suggest a previously unforeseen cost of iridescent plumage traits that may help to explain the evolution and distribution of iridescence in birds. Supplementary material available online at

Iridescent colouration of male Anna’s hummingbird (Calypte anna) caused by multilayered barbules

“…As noted by Durrer (1977), the barbules of bird feathers are usually oriented perpendicular to the feather plane, probably to optimise hydrophobicity (Eliason and Shawkey 2011). To display a structural-coloured barbule, it must be rotated so that it becomes more or less coplanar with the feather surface.…”

Population differences in the structure and coloration of great tit contour feathers

“…Recent studies also suggest that populations may adaptively increase feather melanization when exposed to unfavourable conditions for the plumage (Peele et al ., ). Furthermore, structurally‐based coloration such as in iridescent feathers is known to impair plumage hydrophobicity (Eliason & Shawkey, ). Therefore, understanding microstructure variation in colourful contour feathers requires the consideration of the different kinds of pigments and structural coloration involved.…”

Reduced sexual dichromatism, mutual ornamentation, and individual quality in the monogamous Zenaida dove Zenaida aurita

“…Alternatively, similarity in plumage coloration between the two sexes might simply be a genetically correlated response in one sex, male or female, to selection in the other sex (Lande , Roulin and Jensen , but see Gluckman ). In such a case, one would expect to observe a negative relationship between fitness and the intensity of plumage coloration in one sex and a positive one in the other one (Fitzpatrick et al , Roulin et al ), if color entails production or maintenance costs, which may be the case with iridescent colors (Dyck , Eliason and Shawkey , Meadows et al ). Although we did not assess the consequences of variation in plumage coloration in terms of survival or reproductive output here, we did not detect sex‐related differences in the relationship between plumage coloration and body condition.…”