Under varying prey abundance, generalist consumers should be less affected than specialists due to their more diverse diet. Nonetheless, when prey availability declines, interspecific competition among consumers should increase and could lead to increased intraguild predation. We examined these potential effects in a generalist predator of the tundra, the Glaucous Gull (Larus hyperboreus), over a 7-yr period characterized by large fluctuations in lemming abundance, a potential prey item for gulls. We studied diet by analyzing regurgitated pellets collected at nests and blood nitrogen and carbon stable isotopes, and we monitored annual nesting density and reproductive success on Bylot Island, Nunavut, Canada. Stable isotopes revealed a relatively similar contribution of terrestrial and marine food sources to the gull diet, although the terrestrial contribution increased in the year of high lemming abundance. According to analysis of pellets, diet during incubation was dominated by geese and lemmings, whereas geese were the main prey during chick-rearing. As expected for a generalist predator, annual variation in diet during incubation reflected overall lemming abundance, and the increased consumption of geese during chick-rearing was associated with an influx of goose families into the study area at that time. Gull nest density, laying date, clutch size, and mass gain of chicks did not vary with lemming population fluctuations. Hatching success, on the other hand, was positively related to lemming abundance and was greater for nests located on islets than for those along the shores of ponds and lakes. Intraguild predation on gull eggs by predators such as Arctic foxes, which primarily feed on lemmings during the summer, was probably the main cause of nest failure. Although a generalist predator like the Glaucous Gull has a diversified diet and may feed only opportunistically on lemmings, our results suggest that large cyclical fluctuations in lemming abundance may still affect gull reproductive success through intraguild predation.