The relative influence of physical, chemical, and biotic wetland characteristics on wetland use by spring migrating lesser scaup (Aythya affinis [Eyton]; hereafter “scaup”) is not well understood. We compared characteristics of used and unused wetlands in eastern South Dakota. Used wetlands were larger (\u3e2 ha; P = 0.05), with higher amphipod densities (\u3e500 individuals m-2; P = 0.01) and higher chlorophyll-a concentrations (\u3e0.2; P \u3c 0.05). These wetlands had lower ionic conductivity (1.8 mS; P = 0.02), lower nitrates (1.0 ppm; P = 0.01), lower submerged aquatic vegetation density (P \u3c 0.01), and lower fine sediment proportions (≤150 μm grain size; P \u3c 0.01). Wetland use was best described by amphipod and submerged aquatic vegetation densities. The predictive model explained 50.4% of the variation in scaup use in a reserve dataset. Thresholds of tolerance by amphipods in relation to wetland habitats in the upper Midwest should be investigated further as indicators of a broader range of water and habitat quality characteristics for scaup
