Bird colours in urban mosaics: a study of two passerines

1. 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. 2. One of the best studied examples might be the urban dullness phenomenon, described for carotenoid-based traits, referring to colours being subdued in urban areas. In contrast, melanin-based and structural colouration are still understudied in the context of urbanization. Moreover, much research focused on changes in mean trait expression, while the effect on phenotypic variation, vital from an eco-evolutionary perspective, was rarely studied in this context. 3. Here, we examined urbanization-driven differences in expression and phenotypic variation of carotenoid-based, melanin-based and structural colours of two urban adapters, the great tit (Parus major) and the blue tit (Cyanistes caeruleus). Importantly, birds were sampled across multiple urban and suburban habitats, and replicated in eight Polish cities, located in the under-studied Central and Eastern European region. 4. We found a consistent decrease in mean carotenoid chroma of great tit breast plumage in more heavily urbanized habitats. This trait was also characterized by higher urban phenotypic variation, which could stem from greater environmental heterogeneity in cities. In blue tits, we observed reduced brightness of breast feathers in urban city centres, and an increased brightness of blue tail feathers in more urbanized habitats. 5. Our study sheds light on the complex pattern of colour trait sensitivity to urbanization and emphasizes the need for examining a wider range of species to gain greater insight into the eco-evolutionary processes acting in urban ecosystems.