Urbanisation reshapes ecological and evolutionary processes and often dulls carotenoid plumage. Using 10 years of paired urban/rural data on great tits and blue tits, the study links yellow ventral carotenoid chroma to clutch size, fledged offspring and fledging success. Urban females show stabilising selection with counter-selection against highly pigmented birds; urban great tit males show directional selection favouring duller plumage for fledging success. The most productive urban pairs are less pigmented than average, while rural selection is relaxed. Results suggest natural selection may drive the urban trend toward reduced carotenoid colouration, potentially via altered diet favouring cost-effective duller plumage.

Across 11 years of urban/rural data near Paris, urban males and females have larger black ties (“urban melanism”). In urban habitats, males with smaller ties have higher clutch sizes and fledged offspring; older urban females with larger ties show higher fledging success, suggesting possible detox benefits. Rural ornamentation is not linked to performance. Optimal male–female combinations differ by habitat: successful urban pairs combine an average/bigger female tie with a smaller male tie, while rural patterns are weaker. Findings point to sex-specific, context-dependent selection and reduced sexual dimorphism under urbanisation.

Using sub-meter satellite imagery across six European cities, the study relates impervious cover (100 m around nest boxes) to carotenoid chroma in great tits. Chroma declines with imperviousness; older birds and males are slightly more chromatic than yearlings and females. Slopes are robust to bootstrapping and leave-one-city-out tests. Because feathers measured were moulted the prior year, paler birds likely reflect lifelong urban habitat. This provides continent-scale evidence that carotenoid colour declines along a continuous urban gradient, supporting carotenoid colour as a bio-indicator of urban environmental quality.