Island living reduces ornamental plumage colouration in passerines

Background/Question/Methods

Plumage colouration is an important avian phenotype for inter- and intraspecific communication. visual signals are shaped by various biological and ecological selection pressures. Island systems provide unique opportunities to explore patterns of plumage evolution, due to the relatively small areas, the high degree of isolation, and the consistency among worldwide island conditions. The few studies that have examined plumage colour on islands have indicated a general pattern of reduced brightness, however, this research is limited to either relatively few species or restricted geographic areas. In my study, I tested the hypothesis that island living results in a reduction in bird colouration across the order Passeriformes. I performed a phylogenetically controlled analysis to assess the influence of island habitation on plumage colouration. Chromaticity values were estimated using red-green-blue colour values obtained from photos of Passeriformes and island occupation data was compiled for 5810 passerine species and subspecies.

Results/Conclusions

Compared to mainland passerines, island females had higher overall plumage colouration scores, while island males showed 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. The reduction in red carotenoid-based colour in island passerines may reflect decreased consumption of carotenoid precursors through either low availability within the environment or high intraspecific competition. The reduction in blue structural-based colour suggests a shift in feather nanostructure decreasing brightness through increased keratin cortex thickness and increased incoherent light scattering. The lack of relationship between island living and sexual dichromatism may be explained by similarities in mating systems between island and mainland species, in which monogamous breeding with biparental care practices trend towards monochromaticity. This study provides a strong foundation for future research regardings the roles of island living on the evolution of colour in birds.