Feather corticosterone reveals that urban great tits experience lower corticosterone exposure than forest individuals during dominance-rank establishment

Aggressive interactions between city-dwelling great tits could be more frequent due to higher population densities compared with forest habitats. However, urban great tits had lower levels of corticosterone (stress hormones) in feathers suggesting that competing for the top spots in a flock is less stressful in the city. Although the consequences of urbanization for the physiological health of animals are the focus of much active research, an overlooked aspect is how physiology could be indirectly modulated by the urban environment via changes in intraspecific behavioural interactions, particularly among gregarious species. Both urbanization and the establishment, as well as maintenance, of hierarchical rank position are processes that could incur physiological stress. Measurements of glucocorticoids (GCs) in relation to urbanization, however, have yielded inconsistent results. In most cases, GCs have been measured in blood, offering only a 'snapshot' of an animal's current physiological state. Because circulating GCs are incorporated into growing feathers or hair, measurements of feather/hair GCs offer a longer term measure of stress exposure reflecting the whole period of feather/hair growth. During two calendar years, we collected tail feathers from 188 urban and forest great tits (P. major) across multiple sampling sites and analysed corticosterone (CORT-the main GC in birds) levels, reflecting CORT exposure during the extended period in late summer and early autumn when great tits moult and winter flocks are formed. Urban individuals exhibited consistently lower feather CORT (fCORT) levels than forest birds indicating lower overall exposure to CORT during this period. The lower fCORT levels in urban individuals could represent an adaptation to cope with the more challenging urban environment, physiological constraints on stress axis function or a trade-off between the ability to respond to stressors and predation risk during moult. Despite the expectation that CORT responses to urbanization are highly context-dependent, the spatial consistency of our results and agreement with a multi-population study of fCORT in European blackbirds (Turdus merula) suggests a generalization of the effect of urbanization on CORT exposure during post-breeding moult (i.e. not site- or species-specific).

Automated summary

Aggressive interactions between city-dwelling great tits are more frequent due to higher population densities, but urban great tits have lower levels of stress hormones in their feathers, suggesting that competing for dominance in a flock is less stressful in the city. This study highlights the overlooked aspect of how the urban environment indirectly modulates the physiology of animals through changes in behavioral interactions. Measurements of corticosterone (CORT) levels in feathers show that urban individuals experience lower overall stress exposure during the feather growth period. These findings suggest that urban great tits may have adapted to cope with the challenging urban environment or face physiological constraints on stress response during moult.