Examining the link between relaxed predation and bird colouration on islands

Insular ecosystems share analogous ecological conditions, leading to patterns of convergent evolution that are collectively termed the “island syndrome”. In birds, part of this syndrome is a tendency for a duller plumage, possibly as a result of relaxed sexual selection and the reduced need for species recognition. Despite this global pattern, some insular species display a more colourful plumage than their mainland relatives, but why this occurs has remained unexplained. Here, we examine the hypothesis that these cases of increased plumage colouration on islands could arise through a relaxation of predation pressure. We used comparative analyses to investigate whether average insular richness of raptors of suitable mass influences the plumage colourfulness and brightness across 110 pairs of insular endemic species and their closest mainland relatives. As predicted, we find a likely negative relationship between insular colouration and insular predation whilst controlling for mainland predation and colouration, so that species were more likely to become more colourful as the number of insular predators decreased. In contrast, plumage brightness was not influenced by predation pressure. Relaxation from predation, together with drift, might thus be a key mechanism of species phenotypic responses to insularity.