Evaluating the light environment as a contributor to colour differences among related bird species

Abstract Understanding the diversity of colour in nature has been one of the more elusive evolutionary problems. In the terrestrial environment, comparative analyses have associated differences in colour between species to light environment, background, and receiver perception. However, these account for only a small fraction of colour diversity, and it has been difficult to explain why a certain species is the colour it is. Here we examine colour variation across 12 very similar species of warblers belonging to the genus Phylloscopus, whose general brightness along an achromatic axis has previously been related to light intensity in their habitat. Many of these species also show variation in the colour of several plumage regions, including the wing bar, belly, and white vs. green in the outer tail feathers. We ask if these differences can be connected to the spectrum of light found in the habitat of each species. We find little evidence that contrast between patches and adjacent plumage or colour per se is affected by light environment. We argue that the heterogeneity of light environments experienced within a habitat and throughout the day make it unlikely that downwelling irradiance alone has a direct influence on colour variation. Accordingly, other features must have driven colour evolution. Diversification may be driven by environmental characteristics, such as background, or unrelated to environment altogether, reflecting the possibility that many different variants may effectively stimulate a receiver, and those that appear in a certain species reflect stochastic processes (e.g. mutation) and contingency (form of the ancestor).