Growing in a city: Consequences on body size and plumage quality in an urban dweller, the house sparrow (Passer domesticus)

•Juvenile and adult sparrows were captured at 30 sites that differ in urbanization rates.•Body size and plumage quality were negatively correlated with urbanization.•The impact of urbanization on feather quality was only observed in juveniles.•Urban environments energetically constrain sparrows during their development only. In urban environments, wild vertebrates have to adjust to new environmental challenges (e.g., modified resource availability, increased chemical, noise and light pollutions). However, while the pace of urbanization is constantly increasing, little is known about the ultimate consequences of urban life on the condition of free-living organisms during different life-history stages. In this study, we investigated the influence of urbanization on the condition and non-ornamental feather quality in a common wild bird species, the House sparrow (Passer domesticus). Using a national network of trained ringers, almost 600 juvenile (early post-fledging) and adult sparrows were captured at 30 sites that differ in urbanization rates. To specifically test whether urbanization differentially affect individuals during different parts of the life cycle, we used several proxies for the energetic and nutritional conditions experienced during (1) the developmental period (body size, juvenile feather quality), (2) at the time of capture (body condition), or (3) during the molting period (adult feather quality). Using this methodology, we showed for the first time that urbanization is associated with both reduced body size and feather quality in this urban-dweller species across a large geographical scale. Importantly, only tarsus length and juvenile plumage quality, both determined during development in the nest, were affected by the degree of urbanization. Conversely, body condition and plumage quality did not differ along the urbanization gradient in adults. Our results thus suggest that urban house sparrows could suffer from nutritional deficit during their development while such constraints disappear once the chicks have completed their growth.