Colors of a Rainbow: A Study on the Evolution of Iridescent Rainbow Birds

Abstract

Iridescence results from interaction of light and nanostructures, and can produce bright and saturated colors that are not possible with pigments alone. Birds have adopted such iridescent nanostructures to diversify the breadth of feather coloration. Most iridescent birds appear to show a single hue, but a few birds have evolved iridescent plumage that shows the full spectrum of color from blue to red. This remarkable display of so-called rainbow iridescence is understudied, and though the mechanisms underlying this phenotype have been studied in a few birds, its evolution across the bird phylogeny is poorly understood. We explore the occurrence of rainbow iridescence, using the most up-to-date iridescence database (Nordén et al 2021) and online bird guides for our research. Given the dynamic nature of rainbow iridescence, we defined an "iridescent rainbow” to be any brilliant iridescent signal that shows red, green, and blue from a single viewing angle. Our spectral and phylogenetic analyses reveal that rainbow iridescence has a few independent origins in birds, with constraints on red iridescence as a proposed limiting factor. In spite of this scarcity, iridescent rainbow birds exhibit a wide diversity of rainbow characteristics and underlying nanostructures.