I compared the breeding ecology of Collared Flycatchers (Ficedula albicollis) breeding in natural and artificial nest sites in old-growth, oak-lime-hornbeam stands of the Białowieża Forest, eastern Poland, from 1993 to 1999. There were no significant differences in laying date, clutch size, partial clutch losses, partial brood losses, or number of offspring fledged between pairs breeding in natural cavities and nest boxes; however, predation rates were significantly greater in natural cavities. Predation rates may be higher in natural cavities because, under natural conditions like that of the Białowieża Forest, predators may lack experience with artificial nest sites. These results indicate that studies using box-nesting bird populations should be regarded with caution, especially when they involve predation or predator-prey relationships.