Replicated urban mosaics reveal trait- and species-specific shifts in carotenoid and structural plumage colouration of two passerines

Urban environments are characterized by markedly altered conditions, which can exert a negative impact on the organisms that inhabit them. Among phenotypic traits most sensitive to urbanization is the colourful plumage of birds. One of the best studied examples might be the urban dullness phenomenon, referring to carotenoid-based colours being subdued in urban areas. In contrast, melanin-based and structural colouration are still understudied in this context. Moreover, research on feather colouration has focused on changes in mean trait expression, while the effect on phenotypic variation was rarely addressed. Here, we examined urbanization-driven differences in carotenoid-based, melanin-based and structural colours of two urban adapters: the great tit (Parus major) and the blue tit (Cyanistes caeruleus). In total, we sampled 309 birds on 8 replicated urban–rural gradients across Poland (Central and Eastern Europe)—an understudied geographical region in the field of evolutionary ecology. In great tits, mean breast carotenoid chroma decreased, while its variance increased, with increasing urbanization. This pattern was not observed in the blue tit. However, blue tits exhibited contrasting patterns of brightness along the urbanization gradient: carotenoid-based breast brightness decreased, whereas structurally-coloured tail brightness increased, suggesting different underlying processes across colouration types (e.g., diet, feather wear and pollution). In contrast, all melanin-based traits (tie area, wing and tail feathers) in the great tit were unaffected. Our results highlight the fact that urbanization influences colour traits in a species and trait-specific manner, cautioning against extrapolating across species.