Abstract
White colouration is a common and important component of animal visual signalling and camouflage, but how and why it varies across species is poorly understood. White is produced by wavelength-independent and diffuse scattering of light by the internal structures of materials, where the degree of brightness is related to the amount of light scattered. Here, we investigated the morphological basis of brightness differences among unpigmented pennaceous regions of white body feathers across 61 bird species. Using phylogenetically controlled comparisons of reflectance and morphometric measurements, we show that brighter white feathers had larger and internally more complex barbs than duller white feathers. Higher brightness was also associated with more closely packed barbs and barbules, thicker and longer barbules, and rounder and less hollow barbs. Larger species tended to have brighter white feathers than smaller species because they had thicker and more complex barbs, but aquatic species were not significantly brighter than terrestrial species. As similar light scattering principals affect the brightness of chromatic signals, not just white colours, these findings help broaden our general understanding of the mechanisms that affect plumage brightness. Future studies should examine how feather layering on a bird’s body contributes to differences between brightness of white plumage patches within and across species.
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Acknowledgements
We thank Asritha Nallapaneni, Bill Hsiung, Devi Stuart-Fox, Jennifer Peteya, Ming Xiao, Nick Justyn and various anonymous reviewers for comments. We would like to thank the Field Museum of Natural History (Chicago, IL, USA), American Museum of Natural History, and the University of Akron for providing feather samples. This study was funded by a grant from the Air Force Office of Scientific Research (FA9550-16-1-0331) and the Research Foundation Flanders (FWO G007117N).
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Igic, B., D’Alba, L. & Shawkey, M.D. Fifty shades of white: how white feather brightness differs among species. Sci Nat 105, 18 (2018). https://doi.org/10.1007/s00114-018-1543-3
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DOI: https://doi.org/10.1007/s00114-018-1543-3