Journal
JOURNAL OF THE ROYAL SOCIETY INTERFACE
Volume 3, Issue 11, Pages 777-786Publisher
ROYAL SOC
DOI: 10.1098/rsif.2006.0131
Keywords
structural colour; sexual selection; cowbirds; plumage colour; thin-film
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Iridescent structural colour is found in a wide variety of organisms. In birds, the mechanisms that create these colours are diverse, but all are based on ordered arrays of melanin granules within a keratin substrate in barbules. The feathers of the grackles and allies in the family Icteridae range in appearance from matte black to iridescent. In a phylogenetic analysis of this clade, we identified several evolutionary transitions between these colour states. To describe a possible mechanistic explanation for the lability of plumage coloration, we used spectrometry, transmission electron microscopy and thin-film optical modelling of the feathers of 10 icterid species from five genera, including taxa with matte black or iridescent feathers. In matte black species, melanin was densely packed in barbules, while in iridescent species, melanin granules were arranged in ordered layers around the edges of barbules. The structured arrangement of melanin granules in iridescent species created optical interfaces, which are shown by our optical models to be critical for iridescent colour production by coherent scattering. These data imply that rearrangement of melanin granules in barbules is a mechanism for shifts between black and iridescent colours, and that the relative simplicity of this mechanism may explain the lability of plumage colour state within this group.
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