Wintering Common Loons (Gavia immer) exhibit daily social behavior on Lake Jocassee Reservoir, South Carolina

We investigated both social and foraging plasticity in a population of Common Loons (Gavia immer) that overwinter inland on Lake Jocassee Reservoir, South Carolina, using transects and time-activity budgets from 4 January to 10 March 2017. Loons were observed as either solitary individuals (65%) or in groups (35%, i.e., within <= 5 body lengths of another loon). Loons were more social (i.e., in groups) on transects with a river mouth (40.9% +/- 7.1) than on transects without a river mouth (24.2% +/- 5.4). The mean (+/- SE) dive duration for solitary loons was 71.3 +/- 7.7 s (n = 46, range 15-219 s) and several dives exceeded 2 min (22.2%, 10/46). Iu contrast, 15 foraging flocks were characterized by numerous shallow dives of <20 s. The mean group size (+/- SE) was 6.9 +/- 0.8 individuals and group foraging bouts lasted 8.3 +/- 1.2 min. In addition, Homed Grebes (Podiceps auritus) and Ring-billed Gulls (Larus delawarensis) joined loons forming multispecies foraging flocks, 26.7% and 13.3% of the time, respectively. These data indicate Common Loons wintering at Lake Jocassee are social and exhibit foraging plasticity.