Foraging behavior differs with group size among common loons (Gavia immer) overwintering on a freshwater lake in South Carolina

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Devon M. Jackson, Sarah C. Armstrong, Nakiah L. Dague and John N. Mager, School of Science, Technology, and Math, Ohio Northern University, Ada, OH, James Paruk, Department of Biology, Saint Joseph’s College, Portland, ME, Brooks Wade and Sherry Abts, Jocassee Outdoor Education, Salem, SC

Background/Question/Methods
More Common Loons (Gavia immer) appear to be overwintering on freshwater lakes in southeastern North America. Lake Jocassee is a 7,500-acre reservoir in northwest South Carolina that supports an overwintering population of ~150 loons each year. Many of these individuals exhibit a degree of sociality that is uncommon among loons overwintering in marine environments. Consistent with benefits associated with group foraging in environments where food resources are sparse, we predicted that loons would benefit from larger-sized social groups by spending less time foraging, but would make more dives that were of lower duration. From 1-hour time-activity budgets of 93 focal individuals, we examined the relationship between diving behavior and group size for loons overwintering on Lake Jocassee in 2019 and 2020, and used factorial ANOVA was then used to differences in the proportion of time individuals spent foraging, as well as differences in individual dive rates and dive duration, with group size.

Results/Conclusions
Common Loons spent an average of 48.6+3.8% of their time with conspecific groups (mean group size=4, range =1-13 individuals). We found significant differences in our comparisons between the proportion of time individuals spent foraging (ANOVA F7, 318=2.23, P =0.03), individual dive rates (ANOVA F7,241 =4.07, P =0.0003) and dive duration (ANOVA F7,260=16.20, P <0.0001) and group size. Solitary individuals spent more time foraging, but dove less frequently and made longer dives than social individuals, most likely to feed upon solitary predatory fish that are found in deeper waters. In contrast, individuals in groups spent less time feeding, dove more frequently however, made shorter dives, most likely feeding collectively on smaller schooling foraging fishes near the water’s surface. Though we did not see a significant relationship between group size and percent time foraging, larger groups exhibited higher foraging rates and lower dive durations. These differences in foraging behavior with group size may reflect cooperative and competitive tradeoffs associated with larger groups of loons foraging upon schools of small foraging fishes.