Phenotypic signatures of urbanization? Resident, but not migratory, songbird eye size varies with urban-associated light pollution levels

Urbanization now exposes large portions of the earth to sources of anthropogenic disturbance, driving rapid environmental change and producing novel environments. Changes in selective pressures as a result of urbanization are often associated with phenotypic divergence; however, the generality of phenotypic change remains unclear. In this study, we examined whether morphological phenotypes in two residential species (Carolina Wren [Thryothorus ludovicianus] and Northern Cardinal [Cardinalis cardinalis]) and two migratory species (Painted Bunting [Passerina ciris], and White-eyed Vireo [Vireo griseus]), differed between urban core and edge habitats in San Antonio, Texas, USA. More specifically, we examined whether urbanization, associated sensory pollution (light and noise) and brightness (open, bright areas cause by anthropogenic land use) influenced measures of avian body (mass and frame size) and lateral eye size. We found no differences in body size between urban core and edge habitats for all species except the Painted Bunting, in which core-urban individuals were smaller. Rather than a direct effect of urbanization, this was due to differences in age structure between habitats, with urban-core areas consisting of higher proportions of younger buntings which are, on average, smaller than older birds. Residential birds inhabiting urban-core areas had smaller eyes compared to their urban-edge counterparts, resulting from a negative association between eye size and light pollution and brightness across study sites; notably, we found no such association in the two migratory species. Our findings demonstrate how urbanization may indirectly influence phenotypes by altering population demographics and highlight the importance of accounting for age when assessing factors driving phenotypic change. We also provide some of the first evidence that birds may adapt to urban environments through changes in their eye morphology, demonstrating the need for future research into relationships among eye size, ambient light microenvironment use, and disassembly of avian communities as a result of urbanization. We examined whether there was evidence for phenotypic divergence in body size and eye size within four species of songbirds breeding across the San Antonio metropolitan area. While we found no evidence of phenotypic divergence in body size, we found differences in eye size (smaller eyes in more urban populations) and a negative association between eye size and measures of light pollution (brightness and artificial light at night) in residential but not migratory songbird species. Our findings highlight the sensory environment as an important selective force driving avian responses to urbanization and avenues for future research.image

Automated summary

Urbanization leads to rapid environmental change and the production of novel environments, causing phenotypic divergence in bird populations. This study found that there were no differences in body size between urban core and edge habitats, except for the Painted Bunting which was smaller in core-urban areas due to differences in age structure. Residential birds in urban-core areas had smaller eyes compared to their urban-edge counterparts, influenced by light pollution and brightness.