The U.S. Fish and Wildlife Service and Canadian Wildlife Service incorporate information from annual aerial and ground counts of waterfowl into harvest management strategies. Timing of population surveys is thought to be optimal for early nesting species but may not reliably reflect the status of species with later migration chronology. The purpose of this study was to evaluate effects of survey timing on diving duck and duck brood abundance indices in eastern South Dakota. Findings indicate that timing of aerial breeding surveys occurred too early in eastern South Dakota to accurately reflect diving duck population status because birds had not yet settled into breeding habitats and aerial production surveys did not coincide with peak duck brood abundance. Diving duck abundance from aerial surveys conducted during 10–17 May were higher than indices from ground surveys conducted 2 weeks later because lesser scaup (Aythya affinis), ring-necked duck (Aythya collaris), and bufflehead (Bucephala albeola) were still migrating through eastern South Dakota. Ground-survey estimates of redhead (Aythya americana) and canvasback (Aythya valisineria) abundance exceeded the upper limit of 95% confidence intervals for aerial survey estimates. Results from our second year of study showed an 11-fold decrease in lesser scaup abundance and an 8-fold decrease in ring-necked duck abundance in the two weeks following aerial breeding surveys. Brood abundance of dabbling and diving ducks peaked 1–1.5 months after aerial production surveys conducted in early July. Late-nesting indices calculated from aerial surveys were unrelated (P = 0.21) to number of late-hatching broods, whereas number of broods hatching after aerial surveys was correlated with late-nesting indices from ground surveys (r2 = 0.74, P < 0.01) and with number of July ponds (r2 = 0.77, P < 0.01). We encourage scientists to initiate a more thorough evaluation of survey biases to ensure that trends accurately reflect status of duck populations and to explore the possibility of using a separate late May/early June aerial survey after diving ducks have settled into habitat as a way of assessing breeding population trends for these species.
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Vol. 20 • No. 1