We examined sources of variation in incubation patterns among female Wood Ducks (Aix sponsa), and investigated the effect of female nest attentiveness on incubation period. Data were collected from 44 females (n = 911 days) using temperature data loggers to monitor nest attendance throughout incubation. Mean (± SE) incubation constancy was 86.9 ± 0.6% and incubation period averaged 30.9 ± 0.2 days. Females took an average of two bimodally-distributed recesses per day. Duration of recesses averaged 98.6 ± 3.4 min, but were shorter in the morning than in mid-day or late afternoon. Body mass of incubating females declined 0.68 ± 0.2 g day⁻¹, but there was no relationship between constancy and early incubation body mass or weight change of females. Incubation constancy was not correlated with length of the incubation period. For most females, incubation constancy and recess frequency did not change as incubation progressed. The fact that incubating females only lost an average of 3% of body mass, and constancy was not related to either body mass or length of the incubation period, suggests that females were not constrained energetically. Finally, we propose that the combination of reduced predation risk and the need of neonates to be more functionally mature at hatching has selected for longer incubation periods in Wood Ducks and other cavity-nesting waterfowl.