Urbanisation impacts plumage colouration in a songbird across Europe: evidence from a correlational, experimental, and meta-analytical approach

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.