Data & Script from: Paler within the city: carotenoid-based colouration covaries negatively with the proportion of impervious surface in great tits (Parus major) territories across Europe

ABSTRACT - 329 words

Urbanisation globally reshapes resource availability and exposes wildlife to novel stressors, which leads to various phenotypic differences. Carotenoid-based colouration reflects dietary carotenoid supply and potential allocation trade-offs with physiology. Although carotenoid colourations are widely reported to be paler in urban versus rural areas, such phenotypic responses have not been tested along continuous fine-scale gradients at continental scale, across multiple cities in European countries. We investigated how reflectance parameters of adultthe great tits yellow breast feathers covaries with territory-scale habitat characterisation across Europe, to move beyond common urban–rural contrasts, capture within-city heterogeneity, and get a better understanding at which local habitat features might covary with carotenoid-based colouration variation. Using sub-meter satellite imagery, we quantified impervious surfaces within 100m around nestboxes across six European cities and related this to feather carotenoid chroma, brightness and UV chroma. Overall, carotenoid chroma declined with the proportion of impervious surfaces in the breeding territory across the continent. The slope was robust to bootstrap resampling, leave-one-city-out analyses and when controlling for between cities differences in average carotenoid chroma. In addition, older birds were more chromatic than yearlings and males tended to be more chromatic than females. By contrast, there was no effect of the proportion of impervious cover in the breeding territory on brightness and UV chroma, suggesting that impervious cover is associated to the amount of carotenoid pigment deposited in feathers but not to the overall structural quality of the feather. Considering that carotenoid colouration is linked to dietary intake, more impervious surfaces (with less vegetation) could imply differences in resources availability. Our study is the first continent-scale evidence that carotenoid-based colouration declines along a continuous urbanisation gradient, highlighting this trait as a phenotypic correlate of local imperviousness within cities across Europe. More broadly, these findings highlight the value of fine-scale urban habitat characterisation for understanding phenotypic responses to urbanisation and call for future studies linking territory-scale imperviousness to diet, pollutant exposure and fitness.