Island living reduces ornamental plumage colouration in passerines

Island systems provide unique opportunities to explore patterns of plumage evolution. The few studies that have examined colour on islands have indicated a general pattern of reduced brightness, but this research is limited to restricted geographic areas and relatively few species.Here, I tested the hypothesis that island living results in a reduction in bird colouration across the order Passeriformes. To do so,I performed a phylogenetic comparative analysis with 5810 passerine species. Compared to mainland passerines, island females had higher overall plumage colouration scores, while island males showed had no difference in overall plumage colouration. There was no apparent change in the extent of sexual dichromatism.When I focussed on red and blue colour scores independent of the other colours, I found that both red and blue plumage colours were reduced in island passerines when compared to mainland species. These results may demonstrate a reduction in carotenoid and structural-based plumage in island birds, suggesting a relaxation in sexual selection pressures in island species