Little is known about the behaviour of Common Loon (Gavia immer) during the critical overwintering period, let alone the behaviour of the small, but increasing number of loons that overwinter on freshwater lakes in North America. We examined the diurnal time-activity budgets of Common Loon overwintering on a large reservoir in northwest South Carolina between 2018 and 2020. Similar to previous studies of breeding individuals and individuals that overwinter in marine waters, loons (n = 132) overwintering on this reservoir spent most of their time (52%) foraging. However, we found distinct differences in the activity budgets of individuals associated with their degree of sociality. Solitary birds (individuals spending 0–20% of time within 20 m of conspecifics) spent significantly more time foraging than did those that were either loosely-social (>20–<70% of time within 20 m of conspecifics) or strongly-social (70–100% of time). Although solitary loons made as many foraging dives as social birds, their dives were much longer, likely reflecting dives for larger predatory fish. In contrast, social individuals made much shorter, shallower dives, often foraging on shallower baitfish that they appear to pursue to the water surface and consume collectively. Such findings add to our understanding of loon winter behaviour and raise interesting questions regarding social behaviour and the short- and long-term trade-offs associated with social foraging in this species.