Data & Script from: Sex- and pair-level links between melanin-based ornament and reproductive performance differ between urban and rural great tits (Parus major)

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 stSatus linked to aggressiveness and dominance, furthermore, melanin can bind metals ions, raising the possibility that such traits also confer physiological benefits in polluted habitats as proposed by the urban melanism detoxification hypothesis. Yet the fitness correlates of melanin-based ornaments in cities, and particularly in females, remain poorly understood. We investigated how the expression and reproductive correlates of melanin-based black-tie ornamentation vary in great tits (Parus major), using data collected over 11 years 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 for melanin-based traits. Furthermore, in urban habitats, males with smaller black ties relative to the local population average achieved higher reproductive success in terms of clutch sizes and number of fledged offspring. Among females, ornament size did not predict clutch size or number of offspring fledged, but older urban females with larger black ties had higher fledging success, suggesting possible maternal detoxification benefits. In the rural population, black-tie size was not significantly related to number of offspring fledged and fledging success considering individual analyses for both males and females. Pairwise analyses revealed that optimal combinations of male and female ornamentation for reproductive output differed between habitats, with urban environments that might favour association of a female that displays local population average or bigger black-ties and a male with smaller black ties considering clutch size and fledging success. In the rural population, clutch size seems to be above average along a sexual dichromatism gradient, with no other clear optimal association pattern. Our findings suggest that urbanisation may drive sex-specific and context-dependent selection on melanin-based ornamentation, highlighting the need to consider both sexes and within-pair dynamics in studies of urban evolutionary ecology.