We develop a simple model to explore the conditions under which intraspecific brood parasitism would be evolutionarily stable in a cavity-nesting bird, the Barrow's goldeneye. Our results show that parasitism can be maintained by negative frequency-dependent selection, which is consistent with recent speculations. However, when we include the effects of density dependence, we find a density threshold below which frequency-dependent effects on fitness are negligible but above which frequency dependence plays a prominent role. Patterns consistent with either a mixed evolutionarily stable strategy (ESS) or a conditional ESS can therefore be obtained simply by varying population density. These results may provide an explanation for contradictory patterns found in a previous field study of brood parasitism in goldeneyes. More generally, our results indicate that conclusions about the adaptive basis of alternative reproductive behaviors can be influenced strongly by population demography. Evolutionarily stable strategy theory holds that mixed and conditional ESSs are mutually exclusive, yet we suggest that attempts to test between these alternatives in field studies may meet with limited success if population density varies.