The evolution of metallic luster in plumage

Abstract

Plumage with metallic luster gives rise to some of the most mesmerizing colors in the natural world---including that of the peacock's tail, the vibrant blue of a superb bird-of-paradise and the throat coloration of many hummingbirds. These colors arise from a nanostructure in the feather barbules consisting of arrays of melanosomes (melanin-filled organelles). Apart from their beauty, structural barbule coloration offer a rich set of questions at the intersection of evolutionary biology, optics and visual ecology. In this dissertation, I tackle three key questions. First, I explore the diversity of feather nanostructures that give rise to metallic luster, and try to understand this diversity using optics and evolutionary biology. I find that to produce bright and saturated metallic luster, two key nanostructural features are necessary: a multilayer structure and optimal size of the melanin layers. Birds have achieved this in multiple ways, explaining the diversity in structures. Second, I clarify the terminology surrounding structural barbule coloration by separating two properties that are often confounded: iridescence and metallic luster. I develop a quantitative measure of metallic luster using cross-polarization photography and demonstrate that it is metallic luster, and not iridescence, that sets structural barbule colors apart from other types of color mechanisms. Third, I explore the role of historical contingency on the evolution of metallic luster in plumage. Evolving metallic luster requires the modification of melanosome shape and organization, which likely takes time to evolve. Yet, some clades, like the Cuculidae, exhibit frequent gains and losses of the trait. By describing the feather nanostructures of 21 species in the Cuculidae, I show that species that have lost metallic luster still retain modified melanosome shapes. This provides a mechanism for the frequent gain and loss of this trait in the clade, since only melanosome organization has to re-evolve. These results emphasize the role of historical contingency on the evolution of plumage colors.