Urbanization constitutes one of the most profound forms of land-use change and strongly affects global biodiversity and ecosystem functioning. Expansion of urban areas typically leads to species loss but may also induce more subtle changes in species dynamics through selection or plasticity. Using a dual correlative (field) and experimental (aviary) approach, we here show that free-ranging urban house sparrows in southern France were smaller and lighter than their rural counterparts after allometric scaling, whereas 2 independent indices of nutritional (feather growth rates) and developmental (feather asymmetry) stress did not vary with urbanization. When subjecting these individuals to urban or rural diets in a highly predictable, controlled setting, rural but not urban sparrows decreased their body mass, independent of diet type, to the extent that initial scaled mass differences between urban and rural birds disappeared by the end of the captive period. By integrating field- and aviary measurements of body size and mass with indices of nutritional and developmental stress, we conclude that the lower scaled body masses of urban birds likely reflect a plastic response to predictable food supplies, possibly mediated through predation. Urban environments therefore do not necessarily constitute nutritionally stressful environments for species that typically cohabit with humans, such as house sparrows.