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We studied the relationship between the foraging radius and energy economy of least auklets (Aethia pusilla) breeding in colonies on three islands in the Bering Sea (St. Lawrence, St. Matthew, and St. George Islands). The distance to which auklets commuted on foraging trips varied by more than an order of magnitude (5-56 km), but mean field metabolic rate (FMR) did not vary significantly among birds from the three islands. These observations indicate that allocation to various compartments of time and energy budgets is flexible and suggest that least auklets may have a preferred level of daily energy expenditure that is similar across colonies. We modeled the partitioning of energy to various activities and hypothesize that the added cost of commuting incurred by auklets from St. Lawrence Island (foraging radius, 56 km) was offset by reduced energy costs while foraging at sea. Data on bird diets and prey abundances indicated that auklets from St. Lawrence Island fed on larger, more energy-rich copepods than did auklets from St. Matthew Island (foraging radius, 5 km) but that depth-averaged prey density did not differ significantly between the birds' principal foraging areas. However, previous studies have indicated that zooplankton abundance is vertically compressed into near-surface layers in stratified waters off St. Lawrence Island, which suggests that variation in foraging eficiency may have resulted in part from differences in the accessibility of food rather than its absolute abundance in the water column. Our findings demonstrate why long-distance commuting between colony and feeding areas need not significantly affect total daily adult energy expenditures, even in heavily wing-loaded species such as auklets. Interpretation of the energetic ecology of auklets and other seabirds can strongly benefit from information regarding foraging distributions, diets, and prey accessibility.

Foraging Radii and Energetics of Least Auklets (Aethia pusilla) Breeding on Three Bering Sea Islands