Common loon (Gavia immer (Brünnich, 1764)) foraging patterns and the relative importance of cisco (Coregonus artedi Lesueur, 1818) in the diets of loons were evaluated for the Whitefish Chain of Lakes, a set of cold-water cisco refuge lakes in Minnesota, USA. Environmental DNA metabarcoding of loon fecal samples detected 15 fish species. Yellow perch (Perca flavescens (Mitchill, 1814)), mimic shiner (Notropis volucellus (Cope, 1865)), and cisco were the most prominent prey detected across the study lakes. We observed a shift in fish species consumed, with increases in detections of mimic shiner and cisco DNA among loon fecal samples collected in August and September. In some locations, suitable oxythermal habitat became restricted throughout the summer, forcing cisco into surface waters which potentially increased their vulnerability to loon predation. Conversely, large foraging aggregations of loons were observed during late summer through fall at locations with ample oxythermal habitat and abundant cisco populations. We hypothesize that cisco were sought by loons as a high-calorie prey resource prior to migration. Conservation efforts directed at preserving water quality in important cisco refuge lakes are likely to benefit common loons through enhancement of both the forage base, for resident and migrating birds, and breeding habitat suitability.

Common Loons (Gavia immer) winter primarily in marine coastal areas and utilize a forage base that is poorly defined, especially for offshore areas. Information on dive activity is needed for describing foraging strategies and for inferring prey distribution. Archival geolocator tags were used to determine the wintering locations and dive characteristics of adult Common Loons captured and marked on breeding lakes in Minnesota, Wisconsin, and the Upper Peninsula of Michigan. Among loons that completed fall migration, most wintered in the Gulf of Mexico, with smaller proportions wintering off the southern Atlantic Coast or impoundments in the southeastern United States. Adult Common Loons tended to occupy offshore areas of the Gulf of Mexico and the Atlantic Ocean and, on average, spent about 60% of daylight hours foraging. Dive depths were as deep as 50 m (Gulf of Mexico) and dive characteristics indicated that loons were primarily foraging on benthic prey. Total dive duration, time at maximum depth, and postdive surface intervals increased with dive depths among wintering Common Loons. Our results are expected to contribute to the understanding of the wintering ecology of Common Loons and be useful in informing regional and national conservation planning efforts.RESUMEN. Los colimbos comunes (Gavia immer) invernan principalmente en zonas costeras marinas y utilizan una base para su alimentación que está mal definida, especialmente en las zonas de altamar. La información sobre la actividad de buceo es necesaria para describir las estrategias de alimentación y para inferir la distribución de las presas. Se utilizaron marcadores geolocalizadores de archivo para determinar los lugares de invernada y las características de inmersión de los colimbos comunes adultos capturados y marcados en lagos de cría de Minnesota, Wisconsin y la Península Superior de Michigan. Entre los colimbos que completaron la migración otoñal, la mayoría invernó en el Golfo de México, y en menor proporción en la costa atlántica meridional o en embalses del sureste de Estados Unidos. Los adultos de colimbo común tienden a ocupar zonas costeras del Golfo de México y el Océano Atlántico y, en promedio, pasan alrededor del 60% de las horas de luz buscando alimento. Las profundidades de inmersión alcanzaron los 50 m (Golfo de México) y las características de la inmersión indicaron que los colimbos se alimentaban principalmente de presas bentónicas. La duración total de la inmersión, el tiempo a máxima profundidad y los intervalos en superficie tras la inmersión aumentaron con la profundidad de la inmersión entre los colimbos invernantes. Se espera que nuestros resultados contribuyan a la comprensión de la ecología de invernada de los colimbos comunes y sean útiles para informar los esfuerzos regionales y nacionales de planificación de la conservación.