Morphological basis of glossy red plumage colours

Abstract

Brightly coloured feathers, including the brilliant reds produced by carotenoids, are sometimes shiny in appearance. Gloss is a common property of materials and usually arises through specular reflection from smooth, flat surfaces. However, the production of gloss on red feathers has never been examined. In the present study, we compared the optical and structural properties of glossy and matte carotenoid-based red feathers of multiple species to identify the proximate basis for their glossiness. Although specular reflectance did not differ between glossy and matte feathers, diffuse reflectance was lower in glossy than in matte feathers, leading to a higher contrast gloss. Compared to matte feathers, glossy red feathers had thicker barbs with a flatter and more homogeneous morphology, consistent with expectations, as well as thicker outer keratin cortices. Moreover, glossiness was predicted by a principal component regression using these same morphological traits. We demonstrate that the gloss of carotenoid-based red feathers is produced at least in part by a smooth, flattened barb microstructure and an enhanced nanostructure, illustrating a novel colour-producing interaction that neither pigment, nor microstructure could alone attain. How the ecology and evolution of species with glossy red feather differ from those with typical matte red feathers represent rich areas for future study.

Interactions between colour-producing mechanisms and their effects on the integumentary colour palette

“…Although gloss can vary with angle, it is distinct from iridescence, in which hue itself changes with angle of incidence. Indeed, although many glossy materials are also iridescent [67], others are not [68]. This is in part because, unlike iridescence, gloss can be produced by mechanisms distinct from those that make the colours themselves.…”

Convergent evolution of super black plumage near bright color in 15 bird families

“…However, microscale morphological features at a larger size scale than wavelengths of light can also impact plumage appearance. Barb and barbule shape, smoothness and orientation can produce glossy features in a diversity of birds (Harvey et al, 2013;Iskandar et al, 2016;Shawkey and D'Alba, 2017). Recently, we have demonstrated that microscale optical cavities in the surface of birdof-paradise feathers create structurally assisted light absorptionor super blackthrough multiple scattering (McCoy et al, 2018).…”

Microstructures amplify carotenoid plumage signals in colorful tanagers

“…These feathers may also have internal nanostructural changes, which have been shown to make carotenoid-based colors brighter (Shawkey and Hill 2005). Research on microstructural variation in colorful displays, including sex differences, is expanding rapidly (Lee et al 2009;Iskandar et al 2016;Enbody et al 2017;McCoy et al 2018;. To gain more insight into evolutionary dynamics, we require a complete understanding of the physical basis of color.…”

“…It is often thought that carotenoids are a limiting resource in the immune system; counter to this view, canaries with a mutation knocking out tissue carotenoids show no difference in immune system function (Koch et al 2018). Nano-and microstructures can alter the appearance of carotenoid-colored organisms (Shawkey and Hill 2005;Iskandar et al 2016). Is it possible that microstructural elements to carotenoid coloration in birds complicate the link between signal and individual quality, contributing to mixed research results?…”

“…Nanostructures and pigments produce colors through well-understood pathways (Hill and McGraw 2006), but only recently have researchers begun to describe how microstructures can substantially enhance or alter pigmentary colors. Carotenoid pigments produce reds, oranges, and yellows, but microstructures make the colors glossy or matte (Iskandar et al 2016). The African Emerald Cuckoo Chrysococcyx cupreus produces stunning color with milli-scale emerald mirrors (Harvey et al 2013).…”