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

This article investigates whether morphological traits of the dark-eyed junco (Junco hyemalis) evolve predictably across multiple urban populations in Southern California. By comparing urban populations in Los Angeles, Santa Barbara, and San Diego with nearby nonurban populations, the study found that wing length consistently decreased with increased urban built cover across all populations, while bill shape and size varied between urban populations independently of urbanization metrics or temperature. Specifically, Los Angeles and Santa Barbara juncos had shorter, deeper bills than nonurban juncos, whereas San Diego juncos did not show this pattern. These findings suggest that while some morphological traits may respond similarly to urbanization, others reflect population-specific evolutionary or ecological histories, indicating that phenotypic responses to urban environments are not universally predictable across cities.