| Summary: | Dissertation (Ph.D.) University of Alaska Fairbanks, 2017 Red- throated (Gavia stellata) and Pacific (G. pacifica) loons breed throughout Arctic Alaska, often on adjacent lakes. Despite having similar life histories and distributions, these species have contrasting population trends in Alaska. Red-throated Loon populations have fluctuated dramatically over the past 3 decades, while the population trend of Pacific Loons has remained stable. These species-specific, population-level differences may be related to the foraging behaviors of breeding adults: Red-throated Loons feed on marine fishes using central place foraging behavior, while Pacific Loons feed primarily on freshwater prey captured from their breeding lakes. In this dissertation, I examined how differences in diet composition during the breeding season were associated with individual fitness parameters, namely adult body condition, chick growth performance, and breeding success. I used results from this research to address the hypothesis that interspecific differences in foraging behaviors contribute to the contrasting population dynamics of Pacific and Red-throated Loons breeding in Alaska. Using stable isotopes and fatty acid biotracers of diet, I found that the diet of breeding Pacific Loons was composed of a mix of freshwater and marine prey, and that variation in diet composition was related to the size of the breeding lake. Pacific Loons nesting on small lakes consumed more marine prey, suggesting that small lakes were not profitable foraging habitat, but their use for nesting was enabled by the close proximity of marine foraging habitat. In contrast, Red-throated Loons nested on very small lakes and ate a diversity of marine fishes. Despite differences in diet, both species had similar body condition during late-incubation, indicating that their diets met adult energy requirements, and adults maintained similar lipid reserves despite differences in foraging behavior. I found that Red-throated Loon chicks grew rapidly, yet were only 66% of ...
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