A potential role for parasites in the maintenance of color polymorphism in urban birds.

Urbanization is a major challenge for biodiversity conservation, yet the evolutionary processes taking place in urbanized areas remain poorly known. Human activities in cities set new selective forces in motion which need to be investigated to predict the evolutionary responses of animal species living in urban areas. In this study, we investigated the role of urbanization and parasites in the maintenance of melanin-based color polymorphism in the feral pigeon Columba livia. Using a correlative approach, we tested whether differently colored genotypes displayed alternative phenotypic responses to urbanization, by comparing body condition, blood parasite prevalence and parasite load between colored morphs along an urbanization gradient. Body condition did not vary with urbanization, but paler individuals had a higher body condition than darker individuals. Moreover, paler morphs were less often parasitized than darker morphs in moderately urbanized habitats, but their parasite prevalence increased with urbanization. In contrast, darker morphs had similar parasite prevalence along the urbanization gradient. This suggests that paler morphs did better than darker morphs in moderately urbanized environments but were negatively affected by increasing urbanization, while darker morphs performed equally in all environments. Thus, differently colored individuals were distributed non-randomly across the urban habitat and suffered different parasite risk according to their location (a gene-by-environment interaction). This suggests that melanin-based coloration might reflect alternative strategies to cope with urbanization via different exposure or susceptibility to parasites. Spatial variability of parasite pressures linked with urbanization may, thus, play a central role in the maintenance of plumage color polymorphism in this urban species.