Because nest site competition among cavity-nesting birds is often intense, the practice of pairing nest boxes has been promoted as a way to reduce nest-site competition and allow for the relatively peaceful coexistence of a dominant and a subordinate species within an area. Although apparently effective for promoting the coexistence of Tree Swallows (Tachycineta bicolor) and Eastern Bluebirds (Sialia sialis), it is unknown whether this practice might be effective for other competing species pairs. We conducted an experiment to test whether providing paired identical boxes would promote the adjacent breeding of Eastern Bluebirds and 2 smaller/subordinate cavity nesters: the Carolina Chickadee (Poecile carolinensis) and the Brown-headed Nuthatch (Sitta pusilla). Our box pairs were of 2 types. Our Big/Small pairs consisted of one box with a bluebird-sized (38 mm) entrance hole and another with a 26 mm hole (too small for a bluebird but appropriate for a nuthatch or chickadee). Our other pairs (Big/Big) consisted of 2 boxes with identical 38 mm holes. Boxes within pairs were 10 m apart. If bluebirds are intolerant of subordinate cavity nesters breeding nearby, we would expect to find similar (and low) occupancy by nuthatches and chickadees in box pairs of either type containing bluebirds. If, however, bluebirds simply defend their own nest cavity, we would expect to find similar (and high) occupancy by nuthatches and chickadees in box pairs of either type containing bluebirds. Finally, if bluebirds guard all usable nest boxes in a small area, we would expect to find high occupation of Big/Small box pairs by nuthatches and chickadees and low occupation by these species of Big/ Big box pairs. We found that bluebirds excluded both nuthatches and chickadees from adjacent big-hole boxes but not from adjacent small-hole boxes, indicating that pairing identical boxes does not promote coexistence of bluebirds and these smaller species. Smaller, less competitive species require nest boxes that exclude larger and more competitive species.
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Vol. 131 • No. 2