Disrupted melanization reveals a hidden role of microstructures in green plumage coloration

Bird plumage colors arise from pigmentary absorption and/or structural scattering of light within feather nanostructures. Yet the relative contributions of these mechanisms, which often reside in different feather parts, to overall coloration remain poorly resolved. In the plum-headed parakeet (Psittacula cyanocephala), plucked green feathers regrow as yellow. This color shift can be seen as a 'natural experiment' that allows us to disentangle the optical roles of pigments, and nano- and microstructures in feathers underlying green plumage coloration. Here we use macro- and microspectrophotometry with scanning-, and transmission electron microscopy, and Fourier analysis to assess how variation in melanin deposition within barbs and barbules shapes overall feather reflectance. We found that yellow feathers retain an intact spongy keratin-air nanostructure within the barbs that is capable of producing structural green coloration. In these feathers, decreased melanin deposition increases brightness, mainly in the longer wavelengths, reducing the purity of the green color. Further, we found that feathers are only perceived as green when green barbs are found together with melanized barbules, and that when barbules lack melanin, the overall reflectance becomes more yellow. Together these results illustrate how melanin enhances structural colors, and the interplay of barb and barbule coloration shapes overall feather appearance. Together this allows for the multifunctional benefits of melanin while maintaining bright colors.