Despite its name, the Tree Swallow is a bird of open fields, meadows, and marshes, a species that uses trees only for nesting and occasional roosting. And yet its name is fitting, as no other trait has so singularly distinguished this species as its decision of where to nest. Not diligent builders, Tree Swallows must rely heavily on woodpeckers and other species to excavate and abandon tree cavities in which they can raise their own young. Such cavities are often in short supply, and competition over them has probably shaped much of the breeding ecology and behavior of Tree Swallows, including early spring arrival (Tyler 1942), early breeding and the use of profuse feathers to line the nest (Winkler 1993, Lombardo et al. 1995), intense defense of the nest cavity against conspecifics (Rosvall 2008), and large populations of both male and female floaters (Stutchbury and Robertson 1985).
Researchers long ago discovered that Tree Swallows would readily accept artificial nest boxes. Owing largely to this fact, as well as to the general hardiness and tractability of the species, the Tree Swallow has emerged as something of a model organism in ecology (Jones 2003). Several branches of ecology have benefited from this association, and major advances have been made in the fields of climate change impacts (e.g. Dunn and Winkler 1999), ecotoxicology (see McCarty 2002a), life history theory (e.g. Winkler and Allen 1996, Dunn et al. 2000, Ardia 2005a), mating systems (e.g. Dunn and Hannon 1992, Kempenaers et al. 2001, Whittingham et al. 2006), vertebrate physiology (e.g. Ardia et al. 2003, Haussmann et al. 2005a, Palacios et al. 2007), and behavior (e.g. Stutchbury and Robertson 1987a, Lifjeld et al. 1993, Winkler et al. 2004). As a result, the Tree Swallow stands today as one of the best-studied birds in North America (Jones 2003). Despite this fact, relatively little is known about its migratory or wintering ecology, and studies of breeding populations outside of northeastern North America are sparse.