Use of Clutch Size to Infer Brood Parasitism in Wood Ducks

Total clutch size often is used to infer the frequency of brood parasitism in wood ducks (Aix sponsa), but the accuracy of this method is unknown. By monitoring nest boxes daily, we determined parasitism rates in 35 nests over 2 years and then compared these rates with those that would have been inferred using published clutch size criteria. Clutch size alone gave an inaccurate estimate of parasitism. The criterion «>12 eggs» slightly overestimated (+3%) the true parasitism rate, while «>13 eggs» slightly underestimated it (−8%)