Data & Script from: Paler in the city: carotenoid-based colouration covaries negatively with imperviousness in great tits (Parus major) across Europe

Urbanisation reshapes resource availability and exposes wildlife to novel stressors. Carotenoid-based colouration, reflects dietary carotenoid supply and potential allocation trade-offs with physiology. Although carotenoid coloration is widely reported to be paler in urban versus rural areas, phenotypic responses have not been tested along continuous, fine-scale gradients across multiple cities. We investigated great tits carotenoid-based colouration covariation with territory-scale habitat characterisation across Europe. Using sub-meter satellite imagery, we quantified impervious surfaces within 100m around nestboxes across six European cities and related this to feather carotenoid chroma. Chroma declined with imperviousness, with older birds and males more chromatic than yearlings and females. The slope was robust to bootstrap resampling and leave-one-city-out analyses. Paler birds breed in more impervious areas, which may reflect lifelong habitat choices. This is the first continent-scale evidence that carotenoid-based colouration declines along a continuous urbanisation gradient, highlighting this trait as a bio-indicator of imperviousness within cities.