Nicolas Bekka > Urban Ecologist

Urbanisation is a major driver of environmental change, reshaping ecological and evolutionary processes. Urban-driven phenotypic differences are increasingly documented, but the underlying role of selection is still understudied. One pattern is the consistent reduction of carotenoid-based plumage pigmentation in city birds. Urban carotenoid-based colouration dullness may reflect poorer dietary carotenoid availability and a possible tradeoff between ornamentation and physiological functions. We investigated carotenoid-based plumage colouration covariation with reproductive performance in great tits Parus major and blue tits Cyanistes caeruleus, two passerine species found in urban habitats, with overlapping ecological niches but specific foraging ecology. Using 10 years of data from a pair of urban and rural populations, we tested whether covariations between the yellow ventral feathers carotenoid chroma and three measures of reproductive performance (clutch size, number of offspring fledged, and fledging success) differ between habitats. Our analyses uniquely examines both individual and pair-level data, and test for parallel evolutionary trajectories and habitat-specific selection on carotenoid-based plumage. In both species, carotenoid-based colouration of urban females showed stabilising selection, with counter-selection against highly pigmented individuals. In great tits, urban males exhibited directional pattern favouring duller plumage considering fledging success and opposite urban versus rural tendencies considering the number of fledged offspring. The urban great tit pairs that produced the largest clutches and fledged more offspring consisted of males and females that had less pigmentation than the population average. In contrast, selection patterns in the rural population were relaxed, showing no clear association between pigmentation and reproductive performance in either species. These results match the global trend of reduced carotenoid-based colouration in urban environments, and suggest natural selection may contribute to these divergences. Urban habitats may alter dietary resources, favouring less pigmented plumage as a cost-effective adaptation. These findings invite further investigation both in other coloured traits and across diverse urban and rural populations.

Urbanisation imposes novel selective pressures on wildlife, altering sexual signalling and the pathways by which ornaments map onto fitness. In birds, melanin-based plumage traits often function as badges of status linked to aggressiveness and dominance, evolving through competition and mate choice. Furthermore, melanin can bind metals ions, raising the possibility that such traits also confer physiological benefits in polluted habitats as proposed by the melanistic detoxification hypothesis. Yet the fitness correlates of melanin-based ornaments in cities remain poorly understood, particularly in females. Moreover, how combinations of male and female ornaments relate to reproductive output is currently unknown. We investigated how the expression and reproductive correlates of melanin-based black-tie ornamentation vary in great tits Parus major, using 11 years of data collected in urban and rural sites near Paris, France. Urban males and females exhibited significantly larger black ties than their rural counterparts, consistent with the "urban melanism" pattern and supporting the detoxification hypothesis. Furthermore, in the urban habitat, males with smaller black ties relative to the habitat average achieved higher reproductive success in terms of clutch sizes and number of fledged offspring. Among urban females, ornament size did not predict clutch size or number of offspring fledged, but older females with larger black ties had higher fledging success, suggesting possible maternal detoxification benefits. In the rural habitat, male or female black-tie sizes were not significantly related to reproductive performance. Moreover, optimal combinations of male and female ornamentation differed between habitats. In urban environments, the most successful pairs considering clutch size and fledging success combined an average or bigger black-tie female and a smaller than average black tie male. In the rural population, clutch size tends to be above average along a sexual dichromatism gradient, while no clear optimal association pattern emerged for the number of offspring fledged and fledging success. Altogether, this suggests that urbanisation may drive sex-specific and context-dependent selection on melanin-based ornamentation and favour reduced sexual dimorphism, highlighting the need to consider both sexes and within-pair dynamics in studies of urban evolutionary ecology.

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 combined continent-wide bird colouration data with territory-scale habitat characterisation to ask whether carotenoid-based colouration tracks impervious cover in great tits Parus major across Europe. Using sub-meter satellite imagery, we quantified impervious surfaces within 100 m around the breeding nestboxes across six European cities and related this to carotenoid chroma. Chroma declined with imperviousness, with older birds and males slightly more chromatic than yearlings and females. The slope was directionally robust to both stratified bootstrap resampling (1,000 refits) and leave-one-city-out analyses, indicating it is neither an artefact of uneven sampling nor driven by any single city. Because feathers measured were moulted the preceding year, our results demonstrate that paler birds in urban areas across Europe breed in more impervious areas, and may reflect a lifelong habitat. Together, these findings provide the first continent-scale evidence that carotenoid-based colouration declines along a continuous urbanisation gradient, highlighting carotenoid-based colour as a bio-indicator of environmental quality within cities.