Data & Scripts from: Colours of urban selection: carotenoid-based signals reveal divergent urban/rural evolutionary trajectories in two closely related passerines.

The content of this archive includes data sets, metadata, R scripts, and images associated with the article "Colours of urban selection: carotenoid-based signals reveal divergent urban/rural evolutionary trajectories in two closely related passerines" (N. Bekka et al. 2025, doi: 10.1002/oik.11617). Abstract: Urbanisation is a major driver of environmental change, profoundly reshaping ecological and evolutionary processes. Urban-driven phenotypic differences are increasingly documented, but the potential role of selection underlying those divergent phenotypes is still poorly understood. Colouration change in urban birds is widely observed, especially a reduction in carotenoid-based feather pigmentation, a trait depicting a trade-off between physiological functions and ornamental signalling. This study examines how urbanisation influences carotenoid-based plumage colouration covariation with reproductive performance in great tits (Parus major) and blue tits (Cyanistes caeruleus), two passerine species commonly found in urban habitats, with overlapping ecological niches but specific foraging ecology. Using 10 years of data from urban and rural populations, we analysed the relationship between carotenoid chroma of the yellow ventral feathers of both species and three measures of reproductive performance (clutch size, number of offspring fledged, fledging success). Our findings revealed divergent patterns between habitats. Urban females of both species experienced stabilising selection, with counter-selection against highly pigmented individuals, while urban great tit males exhibited directional pattern favouring duller plumage, considering the number of fledged offspring. Additionally, great tit pairs with less chromatic individuals in both sexes produced larger clutches and fledged more offspring in urban habitats, indicating optimal pairs in terms of reproductive performance combine both parents being less pigmented than the population average. In contrast, selection patterns in rural populations were more relaxed, showing no clear association between pigmentation and reproductive performance. These results align with the global trend of reduced colouration in urban environments, suggesting natural selection is a key driver of these divergences. Urban habitats likely disrupt carotenoid-based signal honesty by altering dietary resources and increasing environmental stress, favouring less pigmented plumage as a cost-effective adaptation. This study suggests the habitat-specific honesty of carotenoid signalling and invites further exploration of correlations between colouration and fitness estimates. Finally, this study highlights the possibility of ongoing parallel evolutionary pathways in urban environments in two passerine species sharing close ecological niches.