We manipulated nestbox choices in Eastern Bluebirds (Sialia sialis) to assess (1) whether the presence of a previously used (and presumably parasite-ridden) nest cavity increases or decreases the likelihood of within-season nestbox reuse and (2) whether birds prefer previously successful cavities. Initially, birds were presented with two clean identical nestboxes erected 1 m apart. After the first nesting, we removed nest material from half of the successful box pairs and recorded subsequent nesting choices. Given a choice between a used and an unused box, bluebirds chose the unused but parasite-free cavity significantly more often. Presented with a cleaned successful box and an identical unused one, bluebirds opted to reuse the former significantly more often. Those results suggest that (1) bluebirds recognize a cost of within-season nest reuse and are willing to switch nest sites to minimize parasitism, (2) bluebirds prefer successful cavities, but only if they are clean, and (3) in our population, in which cost of nest switching was minimized, the aversion to parasites was stronger than the preference for successful cavities.