We investigated the importance of passive sperm loss in the outcome of sperm competition in captive, wild-type Muscovy Ducks (Cairina moschata). In nature, Muscovy Ducks can be expected to experience more intense sperm competition than most other waterfowl because of their non-monogamous mating system. We estimated the instantaneous rate of sperm loss from the reproductive tracts of female Muscovy Ducks as 0.0235 ± 0.0018 h^-1 [SE], a fairly typical rate in comparison to the few other species of birds in which passive sperm loss has been measured. We also measured sperm precedence in trials in which a captive female was allowed to mate with two males in succession, with either a 24-h or a 72-h lag between matings. Paternity was determined with microsatellite markers. The mean proportion of a female's eggs fertilized by the second male (P₂) was 0.72 ± 0.14 in trials with the 24-h lag and 0.42 ± 0.13 in trials with the 72-h lag. The last-male precedence observed in the 24-h trials can be explained by a quantitative model in which passive sperm loss alone determines average success, but this model is not consistent with the outcome of the 72-h trials. Other factors, including perhaps postcopulatory female choice, must be acting in addition to passive sperm loss in the trials with the longer lag.