Habitat shifts and foraging performance of American wigeon in winter

For migratory waterfowl, winter is a time of shorter days, decreased resource availability, and increased costs due to thermoregulation and courtship. Herbivorous waterfowl are especially susceptible to energy intake deficits due to nutritional and physiological constraints. In order to meet the energetic demands of winter, migration, and upcoming breeding season, waterfowl select habitats that maximize foraging performance (i.e., energy intake). In this study, I quantified American wigeon (Anas americana) densities and energy intake rates (kJ min-1) from September 2013 to May 2014 on three habitat types around Arcata Bay, CA: permanent freshwater ponds, tidal bay, and seasonally flooded pastures. I calculated energy intake rates using true metabolizable energy values of food resources, bite sizes, and bite rate. Wigeon densities around Arcata Bay mirrored profitability of available food resources from September 2013 to May 2014, whereby wigeon shifted habitats to maximize foraging performance. Bird densities were highest in September and October when wigeon congregated on freshwater habitats, low densities occurred in mid-winter on tidal habitats, and intermediate densities were experienced in spring on flooded pasture habitats. Corresponding intake rates achieved ranged from 1.43 kJ min-1 in autumn, to 0.39 kJ min-1 in mid-winter, and 2.18 kJ min-1 in spring. These findings are consistent with those for other waterfowl species attempting to meet daily energy demands on winter habitats. Females had a greater energy intake rate than males on freshwater and pasture habitats. Time of day and moon phase had varied effects on individual intake rates depending on habitat type. Tide height did not affect wigeon energy intake rates. Disturbance events occurred on 29 of 388 (7.5%) focal bird foraging observations; 9 due to raptors, 7 to larger waterfowl, and 7 to human activities. Wigeon demonstrated the ability to react to changes in resources across winter and exploit food resources most profitable of those available. Managing for a variety of habitat types provides migratory waterfowl the opportunity to select food resources that will maximize individual energy intake, leading to greater survival and reproductive success.