The traits of urban animals and plants are changing rapidly in response to urbanization and there is a growing understanding of how urbanization affects the mean phenotypes of populations. However, little is known about how urbanization may impact phenotypic variation, an important target of selection that will determine the future evolution of urban populations. We propose hypotheses about how ecological and evolutionary processes may shape variation in urban habitats. To address some of our hypotheses, I use morphological data on great and blue tits (Parus major and Cyanistes caeruleus, respectively) from a collaborative network of researchers across 13 different European cities. We find that urbanization tended to decrease the mean in morphological traits, which supports previous findings of reduced body size in urban organisms. Urbanization, however, tended to increase the variance in morphology; this trend was stronger for females than males and differed across morphological traits. Our results suggest that complex processes may be acting to increase urban phenotypic variation in this instance and highlights the value of examining overlooked biological effects in variance. We hope to encourage urban researchers to compare variation, alongside means, of ecologically relevant traits in their future work.