Summary
Superterritoriality is a spiteful behavior in which an individual’s relative fitness is improved by reducing the fitness of others through inhibiting them from breeding, rather than through an absolute increase in fitness (Verner 1977). Tree Swallows (Tachycineta bicolor) are obligatory hole-nesters, and a single pair can defend additional sites not essential for successful reproduction (Harris 1979, Muldal et al., 1985), thereby preventing conspecifics from using those nest sites. Harris (1979) suggested that the main function of surplus nest site defense in Tree Swallows is to depress the reproductive success of potential breeders. However, Robertson and Gibbs (1982) found that Tree Swallows do not orient their aggressive nest defense in relation to the location of surplus nest sites and concluded that Tree Swallows defend a circular radius around their nest site, which may encompass additional nest sites. A commentary by Harris (1985) indicates that the issue of superterritoriality in Tree Swallows is still unresolved. Here, we argue that the conditions under which superterritorial behavior would evolve do not occur in Tree Swallows.
Methodology
Our study is based on two populations of Tree Swallows at the Queen’s University Biological Station, Chaffey’s Lock, 50 km north of Kingston, Ontario, Canada. The New Land (NL) population consists of about 60 pairs that breed in nest boxes which are distributed over several hayfields. The Northeast Sanctuary (NES) population has about 35 breeding pairs, with both artificial and natural nest sites over shallow water. During 1985 in the NL, we caught 9 birds with injuries that generally consisted of large bare areas on the back of the head and neck, and sometimes around the eye, where the feathers had been stripped off, exposing bruised skin. In one of these cases the injured bird was caught fighting inside a box with another bird. In the past two years, we have found three cases of a dead Tree Swallow with similar injuries inside a nest box. One instance of a female drowning while fighting has also been observed (Tozer, pers. comm.). In early May, fights involving a resident bird and an intruder near nest boxes are common. On three occasions after observing individuals fighting in the grass, we were able to capture the defeated bird by hand as it was unable to fly. Leffelaar and Robertson (1985) have described two cases of females seriously injured during fights. Although this evidence is anecdotal, it does indicate that Tree Swallows are capable of inflicting serious injury during aggressive interactions.