Among-species variation in hormone concentrations is associated with urban tolerance in birds.

As cities expand across the globe, understanding factors that underlie variation in urban tolerance is vital for predicting changes in patterns of biodiversity. Endocrine traits, like circulating hormone concentrations and regulation of endocrine responses, might contribute to variation in species' ability to cope with urban challenges. For example, variation in glucocorticoid and androgen concentrations has been linked to life-history and behavioural traits that are associated with urban tolerance. However, we lack an understanding of the degree to which evolved differences in endocrine traits predict variation in urban tolerance across species. We analysed 1391 estimates of circulating baseline corticosterone, stress-induced corticosterone, and testosterone concentrations paired with citizen-science-derived urban occurrence scores in a broad comparative analysis of endocrine phenotypes across 71 bird species that differ in their occurrence in urban habitats. Our results reveal context-dependent links between baseline corticosterone and urban tolerance, as well as testosterone and urban tolerance. Stress-induced corticosterone was not related to urban tolerance. These findings suggest that some endocrine phenotypes contribute to a species' tolerance of urban habitats, but also indicate that other aspects of the endocrine phenotype, such as the ability to appropriately attenuate responses to urban challenges, might be important for success in cities.