Extra nest site occupancy by tree swallows: do floaters avoid nest sites near settled pairs?

In this study we tested the hypothesis that ENS occupancy by Tree Swallows results from avoidance by floaters of nest sites near settled pairs. To date, most explanations of this behavior have assumed that a differential cost to defending the second site exists, and all have focused on benefits arising from its successful defense (Kendeigh 1941, Harris 1979, Robertson and Gibbs 1982, Muldal et al. 1985, Finch 1990, Dunn and Hannon 1991, Rendell and Robertson 1994). An alternative explanation is that floaters avoid or reject nest sites too close to already settled pairs. Residents may then occupy the extra sites without any increase in their total defense costs. That is, an ENS may be nothing more than a convenient perch from which to observe the primary site. The “avoidance hypothesis” sees ENS occupancy as an artifact of floater behavior rather than as resulting from the resident pair’s defense and predicts that floaters will not make use of the nest sites in question even if the residents’ defense radius is reduced. The hypothesis was noted in passing by Muldal et al. (1985) but has never been tested.