Foraging niche shift maintains breeding parameters of a colonial waterbird during range expansion

Relating the effects of foraging niche variation to reproductive dynamics is critical to understand species response to environmental change. We examined foraging niche variations of the slender-billed gull (Chroicocephalus genei), a nomadic colonial waterbird species during its range expansion along the French Mediterranean coast over a 16 year period (1998-2013). We investigated whether range expansion was associated to a change in chick diet, breeding success and chicks body condition. We also examined whether breeding success and chicks body condition were explained by diet and colonial characteristics (number of pairs, laying phenology, habitat, and locality). Diet was characterized using dual-stable isotopic proxies (δ13C and δ15N) of feather keratin from 331 individuals subsampled from a total of 4154 chicks ringed and measured at 18 different colonies. δ13C decreased and δ15N increased significantly during range expansion suggesting that chicks were fed from preys of increasing ...