Wood duck (Aix sponsa) hens are known to lay eggs in nests of other wood ducks. Low levels of intraspecific nest parasitism can increase the number of ducklings produced, but at high levels, increased numbers of eggs are offset by reduced hatching success and elevated nest abandonment. Such effects have been observed in nest boxes, stimulating interest in the role of intraspecific nest parasitism in natural cavities. We found high rates of nest parasitism (85% of nests) in natural cavities by applying microsatellite genetic markers to examine maternity for clutches laid in floodplain and upland forests. Clutch sizes, number of parasites per nest, and the number of parasitic eggs deposited per nest were greater in tree cavities located in floodplain than in upland forests (35.2%, 63.6%, 75.0% greater, respectively; P < 0.05), although the proportions of nests parasitized were similar (100% of floodplain nests versus 80% of upland nests, P = 0.11). We compared our data to almost 40 years of published data from studies of wood ducks using artificial nest boxes. Clutch sizes in natural cavities were smaller than reported for 3 populations using nest boxes (10.3%, 29.4%, and 49.2% smaller; P < 0.05), except when only floodplain nests were considered (5.0% larger, 11.6% and 28.8% smaller, P > 0.05). Fewer parasites (1.5 ± 0.1 parasites) contributed to nests in tree cavities than in 1 box-nesting population (2.7 ± 0.04, P < 0.05). We suggest that the high parasitism rates observed in natural cavities reflect the accuracy of microsatellite techniques relative to other methods and that conspecific nest parasitism is pervasive among wood ducks, even in natural tree cavities that are more widely dispersed and well hidden than nest boxes. However, despite the large proportion of cavity nests parasitized during this study, parasitism did not severely depress breeding productivity, as observed in some box-nesting populations. Long-term and experimental research on factors contributing to extremely large clutches in nest boxes could improve the efficacy of nest-box programs.