Extra-pair paternity enhances the reproductive fitness of urban Chinese blackbird

Urbanization is a human-induced process of rapid habitat change that can affect the reproductive success of animals. The occurrence of extra-pair paternity (EPP) may be sensitive to changes in urban ecological conditions. In this study, the Chinese blackbird Turdus mandarinus, a common breeding bird in southern Chinese cities, was selected as the focus species to explore the EPP patterns of monogamous birds in developing cities and the relationship between pairs. Over four breeding seasons, 316 Chinese blackbird nestlings and 20 adults in both urban and rural areas were monitored and genotyped for 11 microsatellite loci to determine progeny affinity. We confirmed that both the brood size and hatching rate of EPP nests were significantly larger than those of within-pair paternity (WPP) nests. The body mass, beak length, head length, and body length of 12-day age extra-pair offspring (EPO) were significantly greater than those of within-pair offspring (WPO), suggesting important benefits of EPP. However, in accordance with the analyses of nest ecological variables, we concluded that EPP occurs most often in areas with low urbanization levels, indicating some constraints for this reproductive strategy in cities. Our findings provide new insights about how this growing human-induced landscape change affects mating behaviour in birds, an important aspect in avian ecology and evolution.