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 spe- cies, and the degree to which urbanized gulls depend upon anthropogenic food may be resolved by isotopic bench- marks. Stable isotope ratios ( ä13C and ä15N) were compared between Silver Gulls breeding at a remote, non- urbanized site (Furneaux Island Group, Bass Strait) and those at an urban (Hobart) colony in Tasmania to distin- guish 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. Received 27 Jan- uary 2010, accepted 24 August 2010.