Paler within the city: Carotenoid-based colouration covaries negatively with the proportion of impervious surface in great tit (Parus major) territories across Europe.

Abstract

Urbanisation alters resource availability and exposes wildlife to novel environmental stressors, potentially driving phenotypic divergence. Carotenoid-based plumage colouration is a condition-dependent trait linked to dietary carotenoid acquisition and physiological allocation trade-offs. Although urban populations are often reported to exhibit paler carotenoid colouration than rural populations, these patterns have rarely been tested along continuous fine-scale urbanisation gradients at a continental scale. We investigated how reflectance parameters of the yellow breast plumage of adult Great Tit (Parus major) covary with territory-scale local imperviousness, used here as a broad descriptor of urban habitat structure, across six European cities. Using 838 adult observations spanning 2005-2024 and sub-meter-resolution satellite imagery, we quantified the proportion of impervious surface within 100 m of nest boxes and related this metric to feather carotenoid chroma, brightness and UV chroma. Carotenoid chroma declined significantly with increasing impervious surface cover within breeding territories across Europe, corresponding to an expected decrease of 0.053 chroma units across the observed 0%-88% imperviousness gradient. This relationship remained robust across bootstrap resampling, leave-one-city-out analyses and models controlling for among-city variation in mean carotenoid chroma. Older individuals exhibited higher carotenoid chroma than yearlings, while males tended to be more chromatic and brighter than females. Impervious surface cover was not associated with feather brightness or UV chroma, indicating that urban habitat structure is linked specifically to carotenoid pigment deposition rather than feather structural properties and overall quality. Because carotenoid-based colouration depends on dietary intake, higher levels of impervious cover and reduced vegetation may reflect lower availability of carotenoid-rich resources in urban territories, although imperviousness should be interpreted as a multifactorial habitat descriptor rather than a direct mechanistic proxy. Our study provides the first continent-scale evidence that carotenoid-based colouration declines along a continuous urbanisation gradient. These findings identify plumage carotenoid chroma as a phenotypic correlate of local imperviousness within European cities and highlight the importance of fine-scale habitat characterisation for understanding phenotypic responses to urbanisation. Future studies should test the mechanistic links between territory-scale imperviousness, diet composition, pollutant exposure, physiological condition and fitness.