Urbanisation is increasing rapidly across the globe, transforming landscapes, presenting organisms with novel challenges, shaping phenotypes, and impacting fitness. Urban individuals are claimed to have duller tones in carotenoid-based traits, the so-called ″urban dullness″ phenomenon. However, at the intra-specific level, this generalisation is surprisingly inconsistent and often based on examples from few populations or a limited geographic area. Here, combining correlational, experimental, and meta-analytical results from a common songbird, the great tit (Parus major), we investigated carotenoid-based plumage coloration in urban and forest populations across Europe. We find that, as predicted, urban individuals are paler than forest individuals, although there are large population-specific differences in the magnitude of the urban-forest contrast in colouration. Our meta-analysis indicates a non-significant effect of environmental pollution on carotenoid-based plumage for the species, suggesting that the observed differences across urban populations are not only driven by pollution. Finally, using one region as an example (Malmö, Sweden), we reveal population-specific processes behind plumage colouration differences, which are likely the result of variation in the spatial and temporal distribution of carotenoid-rich resources in anthropogenic environments. Our results provide convincing evidence for consistency on the “urban dullness” phenomenon but highlight that its magnitude depends on local urban characteristics.

Great tit plumage carotenoid colouration (chroma and brightness) measurements from 5 urban/forest population pairs, in addition to cross-sectional data, a cross-fostering experiment for one urban/forest population and a meta-analysis on the effects of anthropogenic stressors (urbanisation and pollution) on great tit plumage colouration.