Presented by Juliet Lamb - Email: firstname.lastname@example.orgHabitat partitioning, wherein species that occupy the same geographic areas select different locations or resources within their habitats, is considered key to the coexistence of closely related sympatric species. However, partitioning can be difficult to observe in natural settings due to the complexity of measuring niche breadth across relevant environmental features. Sea ducks, which form large mixed-species flocks in winter and spatially and temporally overlap extensively throughout the annual cycle, provide an opportunity to determine how habitat partitioning of closely-related species allows coexistence while utilizing shared resources. We used satellite telemetry data and a multivariate habitat selection analysis to investigate habitat partitioning across the annual cycle in five species of sea ducks (Black Scoter, Surf Scoter, White-winged Scoter, Long-tailed Duck, and Common Eider) from throughout eastern North America and the Great Lakes, U.S.A. Our results suggest that niche separation and habitat selectivity are strongest at breeding sites and during the post-breeding molt and migration period, when highly productive habitats are spatially limited and individual mobility is restricted. During winter, when productivity is low throughout the shared range, and during spring, when productivity is uniformly high, species overlap extensively across environmental covariates. We discuss the effects of seasonal variation in environmental conditions on competitive interactions and habitat partitioning, identify key multi-species habitat features throughout the annual cycle, and the associated management implications.