A multiscale physicochemical characterization of melanin-based feather colouration in the rapid capuchino seedeater radiation

Abstract Melanin-based plumage colouration in birds is shaped by pigment composition as well as melanosome morphology and distribution; however, the ways in which these factors together modulate observable colour remain poorly understood. We investigate this relationship in the capuchino seedeaters (genus Sporophila), whose recent, rapid radiation driven by sexual selection resulted in 12 species with diverse colouration patterns. Using scanning electron microscopy and micro-computed tomography, combined with a novel application of Fontana–Masson stain to image melanosomes at high resolution, we characterize melanosome distribution and morphology in several variably coloured plumage patches across capuchino species. Melanosome morphologies followed patch-specific patterns that did not directly correlate with colouration: crown feather melanosomes were larger, more elongated and had greater percent eumelanin content than those in belly, throat or dorsum/rump patches. We also observed that dorsal patches had more total melanin than ventral ones, with pigment and colouration patterns suggesting possible signalling and photoprotective roles. More generally, we show how the patch-specific colouration of male capuchinos is accompanied by differences in melanosome morphology and melanin composition and abundance. Our work highlights the challenges that remain in understanding how the nanoscale mechanisms of melanin-based pigmentation translate into macroscale plumage colouration.