Summary
Tree Swallows (Tachycineta bicolor) are hole nesters that are often limited by nest site availability (Holroyd, Can. Field-Nat. 89:60-64, 1975). Populations generally increase following the erection of nest boxes (Low, Bird-Banding 3:76-87, 1933; Chapman, Bird- Banding 6:45-57, 1935; Holroyd 1975), and competition for nest-sites occurs early in the breeding season (Kuerzi, Proc. Linn. Soc. N.Y. 52-53:1-52, 1941). Territorial defence of the area surrounding the nest-site may further limit the availability of breeding opportunities if more than one nest-site is defended by a single pair of birds. Harris (Can. J. Zool. 57: 2072-2078, 1979) argued that the defense of unnecessary resources (extra boxes) was an example of superterritoriality (Verner, Am. Nat. 11 1:769-775, 1977). Robertson and Gibbs (Condor 84:313-316, 1982), however, found that territorial defense was not focused on extra boxes and therefore has probably evolved as the most effective way to defend the single nest-site required for breeding, rather than as a superterritorial strategy.
Clearly, nest-site limitation leads to aggressive interactions among Tree Swallows, and territorial defense can prevent conspecifics from breeding even when unoccupied boxes are present (Harris 1979). It is generally assumed that competition for nest-sites involves defense of the nest by both members of the pair (Harris 1979, Robertson and Gibbs 1982). Here, we report several instances of nest-site usurpation by females. These indicate a lack of cooperative defense by the pair and suggest that females may be competing for breeding opportunities involving a male with a nest-site, rather than a nest-site per se.
Methodology
During the summer of 1982, we studied the breeding ecology of Tree Swallows at Queen's University Biological Station near Chaffey's Lock, Frontenac Co., Ontario. The study site consisted of four hayfields and a nearby pond. There were 139 nest boxes distributed over this area. In three of the fields and in the pond some of the boxes were arranged in spirals, so that there were boxes 1, 2, 4, 8, and 16 m away from a central box (see Robertson and Gibbs 1982). Additional boxes were interspersed between these spirals with a minimum interbox distance of 16 m. A total of 47 clutches hatched, and an additional eight clutches were incomplete, abandoned, or infertile. The nest box occupancy rate for the entire season was 40% (55/139). Both early in the season, while nest-sites were being established, and later during the incubation and nestling periods, interactions between members of resident pairs, and between resident pairs and intruders, were observed. Eighty-four percent of the breeding females and 46% of the males were color-marked for individual identification.