Interspecific competition over nest sites is common among cavity-nesting birds, but little is known about what determines the outcome of such contests, particularly whether or not prior ownership plays a role. Using a box removal and replacement experiment, I tested whether Mountain Bluebird (Sialia currucoides) or Tree Swallow (Tachycineta bicolor) had the higher resource holding potential (RHP). A different box-blocking experiment with paired boxes on territories measured the extent to which prior ownership influenced contest outcomes for both species. Behavioral observations in the pre-laying period showed more physical aggression by Mountain Bluebird against Tree Swallow. Nevertheless, Tree Swallow won 70% of boxes when neither species had prior ownership, suggesting Tree Swallow had a higher RHP. When their boxes were blocked, 24% of Tree Swallow usurped boxes from Mountain Bluebird, which did not differ statistically from the 33% of boxes usurped by bluebirds from swallows; thus, prior ownership does not guarantee winning the contest for either species. Currently, Mountain Bluebird arrives earlier in spring than does Tree Swallow and relies more extensively on prior ownership to retain nest sites. This advantage could be jeopardized if the migration schedule of Tree Swallow is accelerated due to climate change, and so it is important to understand the role of prior ownership in contests for nest sites for birds.