Urbanization of the silver gull: Evidence of anthropogenic feeding regimes from stable isotope analyses

Access to human-derived food is thought the major cause of population increases in many gull species, and the degree to which urbanized gulls depend upon anthropogenic food may be resolved by isotopic benchmarks. Stable isotope ratios (δ¹³C and δ¹⁵N) were compared between Silver Gulls breeding at a remote, nonurbanized site (Furneaux Island Group, Bass Strait) and those at an urban (Hobart) colony in Tasmania to distinguish potential differences in feeding regime. Analyses of whole blood stable isotopes revealed that non-urbanized gulls tended to have a mixed diet from several sources, while urban gulls fed on a separate food web from and a more freshwater origin. No differences in the stable isotope ratios were detected between sexes or among breeding periods. Birds from Hobart tended to feed at a higher trophic position after egg-laying than before, and reflected a change in food preference. These results provided critical baseline data to measure the degree of urbanization of Silver Gulls in Tasmania in order to study potential health impacts of anthropogenic food on birds.