We investigated temporal and spatial variation in the diet of the Common Murre Uria aalge, the most abundant, locally breeding seabird of the central California continental shelf. We collected murres in coastal, mid-shelf, and outer-shelf waters of the Gulf of the Farallones during the murres' pre-breeding (March-April), breeding (May-August), and wintering (September-February) periods, 1985-1988, Diet samples formed persistent groups as a function of these six spatio-temporal combinations of murre foraging habitat and life-history periods. Temporally, diets varied on a seasonal and interannual basis, with diets during winter and El Nino periods being the most disparate. Spatially, diets differed among the three habitats, independent of time. During the pre-breeding season after the onset of upwelling, euphausiids and juvenile rockfish (Sebastes spp.) became prevalent in the diet. Diets were least diverse during the breeding season because of the dominance of rockfish, especially among murres foraging in mid- and outer-shelf habitats. Other important prey were also significant for commercial or sport fishing: Engraulis mordax and Clupea harengus in coastal waters, Merluccius productus and Loligo opalescens in mid- and outer-shelf waters, and surfperch Cymatogaster aggregata and Brachyistius frenatus in coastal and outer-shelf waters (near reefs), respectively. Results support a hypothesis that diet varies as a function of where murres forage. Hence, if the most characteristic prey of one habitat disappears, murres switch foraging areas, bringing a switch in diet. Owing to environmental changes in the region, murres may be losing the option of prey switching as a strategy to maintain an adequate intake of food.
