As urban areas continue to expand globally, understanding how and why species respond to novel habitats becomes increasingly important. Knowledge of the mechanisms behind observed phenotypic changes in urban animals will enable us to better evaluate the impact of urbanization on current and future generations of wildlife. Physiological changes, such as those involved in the endocrine stress response, may allow individuals to inhabit and thrive in urbanized areas, but it is currently unknown how these changes arise in natural populations. In this study, we performed a 4-way crossfoster experiment in free-living house wren chicks, Troglodytes aedon, to disentangle whether differences in baseline corticosterone between urban and rural individuals is a result of genetic and/or plastic mechanisms during development. We found that urban chicks already had higher corticosterone levels than their rural counterparts on the day they hatched, which suggests a possible genetic component to the corticosterone phenotype. However, rural offspring that were moved to an urban environment significantly increased their corticosterone levels, mimicking those of urban offspring. Our findings suggest that, although differences in baseline corticosterone concentrations between urban and rural individuals may have a genetic component, plasticity plays a pivotal role and can modify the corticosterone phenotype in response to the environment experienced in the first two weeks of life.