There are large numbers of reproductively mature female tree swallows (Tachycineta bicolor) which do not breed due to limits of suitable nesting cavities. Many of these floaters are one-year-old females that have a distinctive subadult plumage. This study examines the behavioral tactics that these subadult female floaters use to obtain breeding opportunities. Early in the season, subadult floaters tended to intrude briefly (Fig. 4) on many nest sites in succession (Figs. 2, 3), although they rarely gained close access to nest sites (Fig. 5). Subadults responded very quickly to vacant nest sites, where the resident female had been experimentally removed, by entering the nest cavity and defending it from conspecifics. We argue that the early season exploratory behavior increases a subadult's chances of discovering a vacant nest site, rather than increasing its success in evicting resident females or laying eggs in other females' nests. During the nestling period, subadult females intruded on fewer nest sites for longer periods, and often gained close access to the nest site. Late in the season, subadult floaters may be gathering information on the quality of nest sites for the next breeding season, rather than searching for current breeding opportunities. The reproductive tactics of subadult female tree swallows are consistent with the breeding threshold model for the evolution of delayed plumage maturation in passerines.
This study was conducted at the Queen's University Biological Station, near Chaffey's Lock, 50 km north of Kingston, Ontario, during the summers of 1984 and 1985, with some additional data from 1982, 1983, and 1986. The two tree swallow populations studied were at the New Land (NL) and Northeast Sanctuary (NES) sites. These populations are 10 km apart and there is very little movement of banded individuals between populations. Both populations have been established for at least 8 years, and have remained at a relatively constant size since 1982, with 55-65 pairs in the NL, and 30-40 pairs in the NES.