Complex patterns of morphological diversity across multiple populations of an urban bird species

Urbanization presents a natural evolutionary experiment because selection pressures in cities can be strongly mismatched with those found in species' historic habitats. However, some species have managed to adapt and even thrive in these novel conditions. When a species persists across multiple cities, a fundamental question arises: do we see similar traits evolve in similar novel environments? By testing if and how similar phenotypes emerge across multiple urban populations, we can begin to assess the predictability of population response to anthropogenic change. Here, we examine variation within and across multiple populations of a songbird, the dark-eyed junco (Junco hyemalis). We measured morphological variations in juncos across urban and nonurban populations in Southern California. We investigated whether the variations we observed were due to differences in environmental conditions across cities. Bill shape differed across urban populations; Los Angeles and Santa Barbara juncos had shorter, deeper bills than nonurban juncos, but San Diego juncos did not. On the other hand, wing length decreased with the built environment, regardless of the population. Southern Californian urban juncos exhibit both similarities and differences in morphological traits. Studying multiple urban populations can help us determine the predictability of phenotypic evolutionary responses to novel environments. Do different populations of animals across multiple cities have similar morphologies across similar, evolutionarily new environments? We looked at the morphologies of an urban songbird, the dark-eyed junco, across three Southern Californian cities and three wildlands. One trait-wing length-was shorter in more urbanized environments. Bill shape, on the other hand, differed between wildlands and cities, but there were differences unique to certain cities. These findings suggest that urban populations can have different responses across urban environments and raises questions as to why and how urban populations phenotypically vary across cities.