Female wood ducks (n = 72) lost on average 1.3 ± 0.1 g/day during incubation. Change in body mass varied greatly among females (CV = 93%); it ranged from individuals that gained body mass to those that lost > 4 g/day. Loss of body mass was directly related to body mass of females in early incubation, but not to time of nesting, age, or clutch mass. Incubating hens lost mass at a greater rate in 1986 (1.9 ± 0.2 g/day) than in 1987 (1.2 ± 0.2 g/day), possibly because drought conditions in 1986 reduced foraging areas. For individuals sampled in both years of the study (n = 14), the group average of early incubation body mass and rate of mass change during incubation were similar between years. However, while similarity of body mass for individual females at the start of incubation was high between years (repeatability 82%), it was low for body mass changes that occurred during incubation (repeatability 31%). Thus, although change in body mass was related to the body mass at which hens began incubation, other factors must also have been involved.