Effective use of seabirds in ecotoxicology monitoring programs ideally requires detailed knowledge of their ecology. Environment Canada recently expanded the Great Lakes Herring Gull Larus argentatus Monitoring Program to a national contaminants monitoring program, utilizing the Glaucous-winged Gull L. glaucescens on the west coast of Canada. The utility of the Glaucous-winged Gull as a marine monitoring species hinges on its consumption of a marine-based diet; however, there is a lack of recent and reliable diet data for this species. Using conventional analysis, we studied dietary ecology at two monitored colonies to elucidate adult diet before egg laying and during incubation, to investigate intra-colony dietary shifts over the breeding season, to examine inter-colonial dietary variation, and to compare findings with historical studies from the early 1970s and 1980s. Results indicate that breeding gulls forage in an opportunistic manner, with marine prey sources predominant at all colonies and breeding stages, but with a wider variety of prey types consumed in locations close to urban development. Chicks at both colonies were provisioned primarily with fish; however, variation in chick diet between 2009 and 2010 indicates that diet can vary considerably on a short time scale. The occurrence of fish fed to chicks appears to have shifted composition from herring Clupea pallasii in the 1980s to primarily Pacific Sand Lance Ammodytes hexapterus at both colonies in 2009 and 2010. Compared with historical records, gulls consumed fewer anthropogenic items and more fish in the Strait of Georgia, whereas diet off the west coast of Vancouver Island appears to have been consistently marine.
Keywords: Glaucous-winged Gull, Larus glaucescens, diet, conventional diet analysis, biological monitoring, historical variation