| Summary: | White-winged Scoters (Melanitta fusca) and Surf Scoters (M. perspicillata) are often assumed to rely on similar marine resources. To evaluate the accuracy of this assumption, we contrast seasonal distributions, foraging effort, and indicators of energy status (body mass and composition, plasma metabolites) in three major foraging sites in Puget Sound, Washington, for these rapidly declining sea duck congeners. For Surf Scoters, distributions and energy status indicated that a mussel-dominated site was relatively important in early winter, but that importance shifted during late winter and spring to seagrass sites that provided either herring spawn or epifaunal invertebrates. As winter progressed, movements among foraging sites and increased foraging effort by Surf Scoters were accompanied by greater variability in their energy status compared with White-winged Scoters; body mass declined over winter by >9% in about one-third of past studies for Surf Scoters, well above the range of losses observed in White-winged Scoters. For White-winged Scoters, lower variability in energy status, foraging effort, and distributions throughout winter suggests that they are better able to regulate energy balance regardless of changing foraging conditions. Greater resistance to seasonal environmental changes in White-winged Scoters may be related to their >50% larger body size, which confers lower mass-specific energy costs and access to a wider size range of bivalve prey. Perhaps because of their greater sensitivity to winter foraging conditions, Surf Scoters appear to rely on a broader range of foraging sites than White-winged Scoters.
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