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.