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.

Impact of urban environment and host phenotype on the epidemiology of Chlamydiaceae in feral pigeons (Columba livia).

Chlamydiaceae are obligate intracellular Gram-negative bacteria found all over the world and known to cause various forms of disease in animals and humans. Urban pigeons are known to be an important reservoir of Chlamydia psittaci, the agent of human psittacosis. In this study, we examined the influence of pigeon houses used to regulate pigeon populations and of melanin-based coloration on several epidemiological parameters of Chlamydiaceae in 708 urban pigeons in Paris. We also identified species and genotypes of Chlamydiaceae present in Parisian populations. First, our results revealed that pigeons roosting and breeding in pigeon houses were equally infected by Chlamydiaceae as those that did not. Second, we found that dark melanic pigeons excreted more Chlamydiaceae than pale melanic ones. Finally, species and strain diversities were very low: all samples were of C. psittaci genotype B. Nevertheless, two atypical Chlamydiaceae were identified based on 16S rRNA and ompA sequences. Our study thus highlights the importance of considering environmental and host phenotype when investigating the epidemiology of infectious diseases.