By nesting in cavities low to the ground, secondary cavity nesters increase the likelihood of detection by mammalian and reptilian predators. By reusing soiled nests, such birds incur the costs of nest ectoparasites. By experimentally varying these two factors, we tested the relative importance of each to nest site fidelity in Eastern Bluebirds (Sialia sialis). Choice experiments demonstrated that bluebirds avoid reusing soiled boxes (if also given a clean box) but prefer to reuse successful nest boxes (if they are clean). Bluebirds also showed a significant preference for high (1.75 m) vs. low (1.5 m) boxes for their first nests of the year, suggesting sensitivity to predation risk. For second nests, we forced bluebirds to choose between their successful soiled box and the unused box from their first nesting attempt. Bluebirds reused soiled high boxes at a significantly higher rate than they reused soiled low boxes, suggesting that bluebirds are willing to incur the costs of parasitism in order to lessen susceptibility to predators.
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