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We conducted the first validation of the deuterium dilution method as a nonlethal technique for estimating the body composition of a shorebird during fall staging as birds fatten in preparation for their migration. For this validation, we captured male (n = 12) and female (n = 8) American Woodcock (Scolopax minor) in the fall of 2018 and 2019 in southern New England, USA. We developed predictive models for estimating body composition of woodcock given each bird's body mass and deuterium space, a proxy for total body water space, estimated using the deuterium dilution method. Accuracy and precision of these predictive models were assessed by comparing the model predictions with directly measured body composition. The top models predicted whole-body lean and fat mass with a relative error of 1.37% and 11.26%, respectively. In comparison, conventionally used morphology-based condition indices were poor predictors of fat mass in fall-staging woodcock. We applied this method to accurately estimate body composition dynamics of woodcock during fall staging, a period of fattening in the annual cycle that is pertinent to subsequent migratory movements, phenology, survival, and habitat management for woodcock. This nonlethal technique also has broad applicability to other migratory birds that show substantial changes in body composition as they prepare for key life-history events such as migration. [ABSTRACT FROM AUTHOR]