GEOGRAPHIC, TEMPORAL, AND AGE-SPECIFIC VARIATION IN DIETS OF GLAUCOUSGULLS IN WESTERN ALASKA

We collected boluses and food remains of adult Glaucous Gulls (Larus h yperboreus) at or near nests and chicks, and digestive tracts from adu lts at three sites on the Yukon-Kuskokwim Delta, Alaska that differed in proximity to marine and terrestrial foods. We observed both geograp hic and temporal variation in diet; gulls consumed proportionately mor e terrestrial prey after peak hatch in late June, and gulls near the c oast consumed proportionately more marine prey than gulls at two inlan d areas. Goslings occurred in > 60% of all samples from these inland a reas. We compared these data to those from a previous study in western Alaska and found no marked differences. Evidence for similar patterns of geographic and temporal variation in diet was found using measurem ents of stable-carbon and nitrogen isotopes in gull and prey tissues. Stable isotope analysis further revealed that adult gulls consumed pro portionately more marine prey (saffron cod, Eleginus gracilis) than th ey fed to their young. Using isotopic models, we estimated that 7-22% and 10-23% of the diet of adult and juvenile Glaucous Gulls, respectiv ely, was comprised of terrestrial species. In addition to significant age-related variation, dietary estimates varied among geographic areas and between pre-and post-hatch periods. Overall, our isotopic estimat es of the contribution of terrestrial prey to the diet of Glaucous Gul ls was less than what may be inferred from conventional methods of die t analysis. Our study emphasizes the benefit of combining stable-isoto pe and conventional analyses to infer temporal and geographic changes in diet of wild birds and other organisms.