| Summary: | Along the Pacific coast of North America, White-winged Scoters (Melanitta fusca) winter in sites that vary widely in habitat conditions. This habitat variation likely alters the relative costs and benefits of using specific sites in terms of factors such as foraging conditions, degree of predation danger, and thermoregulatory costs. To assess how White-winged Scoters adjust their physiology and energy management in response to variation in habitat conditions, we contrasted overwinter dynamics in several physiological indicators across five sites in British Columbia and Washington. We tested the relative support for various hypotheses that considered exposure to wind and waves, water depth, predation danger, diet composition, and collection period as possible underlying causes of variation in physiological indicators. Total body mass and lipid mass were highest at an exposed offshore site, which may reflect an adaptive response to buffer against unpredictable foraging conditions and increased thermoregulatory costs. At nearshore sites where exposure was lower, scoters maintained lower lipid masses throughout the winter. Total lipid mass declined through the winter in all sites, a result consistent with findings for many waterfowl species. However, levels of plasma metabolites (triglycerides, β-hydroxybutyrate) varied little by site, suggesting that, irrespective of body composition, birds at all sites maintained physiological homeostasis. Digestive morphology was strongly related to diet, with smaller gizzards and longer intestines associated with a greater fraction of soft-bodied foods in the diet. Our results suggest that the physiology and energy management of wintering White-winged Scoters is related to local habitat conditions.
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