Floater males engage in extrapair copulations with resident female tree swallows

Floaters are sexually mature yet nonbreeding, nonterritorial individuals (Smith 1978). They are common in many passerine species and avian social systems (see Zack and Stutchbury 1992) but relatively little is known about them. Floating behavior ultimately arises through an excess of sexually mature individuals trying to procure a limited number of nesting sites (Brown 1969, Smith and Arcese 1989). It has been described as an alternative reproductive strategy to territoriality (Austad 1984) and as a behavior based on quality differences among individuals (Smith and Arcese 1989). Floaters are characterized by their frequent intrusions into occupied territories to search for nesting sites (Arcese 1987, Stutchbury 1991) or to attempt extrapair copulations with resident females (Dunbar 1982). In the latter case, males potentially could gain reproductive success without the costs of territory defense and parental care. However, this latter scenario is thought to be unlikely because male floaters typically are deemed to be inferior in quality to resident males (Brown 1969, Smith 1978)