Previous efforts to estimate the daily energy expenditure of American Black Ducks (Anas rubripes) have relied upon diurnal behavior observations or predictive allometric equations. American Black Duck behavior was quantified during morning crepuscular, diurnal, evening crepuscular and nocturnal periods to create a 24-hr time-energy budget. Behaviors and energy expenditure differed between periods and months, with hourly energy expenditure highest during the morning crepuscular period and lowest during the nocturnal period. Daily energy expenditure estimates based on a 24-hr time-energy budget were lower than estimates calculated from extrapolated diurnal behavioral data as well as a predictive allometric equation that uses a generalized dabbling duck resting metabolic rate. However, there was no difference between the 24-hr time budget and a predictive allometric equation using American Black Duck specific resting metabolic rates. Future researchers and managers should acknowledge the assumptions of each methodology to estimate daily energy expenditure when using a bioenergetic approach to estimate carrying capacity.