NEST-SITE FIDELITY IN EASTERN BLUEBIRDS (SIALIA SIALIS) DEPENDS ON THE QUALITY OF ALTERNATE CAVITIES

Abstract Secondary cavity nesting birds have been predicted to switch their nesting location when the quality of the nest site declines such as from accumulation of soiled nest material and parasites. We hypothesized that nest-site switching in Eastern Bluebirds (Sialia sialis) should depend not only on the condition of the original nest, but also on quality of alternate nest cavities. We tested that hypothesis experimentally by providing Eastern Bluebirds with attractive and unattractive nest boxes (as determined from a choice experiment). In one experiment, bluebirds were forced to choose between their soiled box of the preferred type and a clean box of the less-preferred type. Faced with that decision, a significant proportion of pairs opted to reuse their nest cavity, despite the ectoparasitism costs of that option. When given the choice between their soiled box of the preferred type and an identical but clean box, a significant proportion switched to the clean box, which suggests that soiled nests present some cost. By manipulating box type, we were able to decrease and increase, respectively, the costs of nest-site fidelity. Thus, bluebirds do not simply operate under a rule of thumb “switch nest sites when nest quality falls below point x.” Rather, they must weigh the declining quality of their current nest against the relative quality of all other available nest sites.