We investigated the effects of body mass of incubating female Wood Ducks (Aix sponsa) on aspects of their current and future reproduction, and we examined factors that affect length of the incubation period. During three breeding seasons, body mass of female Wood Ducks averaged 578.0 g early and 553.3 g late in the incubation period. Body mass at the start of incubation was not related to either hatching success or length of the incubation period. In one of three years, females that were heavy at the end of incubation survived better to the next breeding season than those that were light. Reduced survival of light females in one year coincided with a greater loss of body mass in that year relative to other years, which indicates that incubation can be an important reproductive cost to female Wood Ducks. There were no relationships between body mass at the end of incubation and date of nesting or clutch size in the next breeding season. Partial correlations between clutch mass and length of incubation that controlled for date of nesting indicated a positive association between clutch mass and incubation length in every year. This relationship was evident only for parasitic nests (i.e. nests in which more than one female was laying eggs). Increased length of the incubation period associated with larger clutch mass represents a potential cost of intraspecific nest parasitism not previously recognized.