Summary
Delayed plumage maturation in males is relatively common among North American passerines, but the Tree Swallow (Tachycineta bicolor) is one of few species in which 1-yr-old females have a distinct subadult plumage. Although they are reproductively mature, most subadult females do not breed in their first year because of intense intrasexual competition for nesting sites. Early in the season, subadult female floaters explore for recently vacated nest sites. The subadult plumage of young females could be adaptive by communicating their low threat to residents, thereby decreasing the cost of this exploration.
To determine whether resident aggression depends on intruder color, we observed live intrusions and conducted model presentations. When the resident female was out of sight or did not respond to intruders, resident males were significantly less aggressive toward subadult females than toward adult intruders in both the nest-building/egg-laying and incubation stages. Early in the season, resident females were equally aggressive toward subadult female and adult intruders. When presented simultaneously with adult and subadult female models, resident males were always more aggressive toward the adult model, whereas females were aggressive toward either model.
We propose two hypotheses for the adaptive significance of subadult plumage in female Tree Swallows: subordinance signaling and sex signaling. Our results suggest that subadult females may reduce resident aggression by signaling their female status to resident males, rather than by signaling their subordinate status to resident females.
Methodology
The study was done at the Queen's University Biological Station, near Chaffey's Lock, 50 km north of Kingston, Ontario, during the summers of 1984-1986. Tree Swallows were studied at the New Land (NL) and Northeast Sanctuary (NES) study areas. These populations were about 10 km apart and had been established for at least 8 yr. Throughout this study, the NL had 55-65 breeding pairs, and the NES had 30-40 pairs. The NL had grids of nest boxes mounted on aluminum posts that were distributed over several hayfields with a total area of 10 ha. The NES had both natural nest sites and nest boxes over about 5 ha of shallow, open water.
In all years the contents of each nest site were checked every 3-4 days to determine the nesting stage of the resident pair. Birds were caught with mist nets or nest traps (Stutchbury and Robertson 1986), and each was banded with a Canadian Wildlife Service numbered band and sexed by the presence of a brood patch (female) or a cloacal protuberance (male), or by its behavior at the nest (Cohen 1984). Females with completely iridescent blue plumage on their upper- parts were defined as adults, and females with brown- blue plumage on their upperparts were defined as subadults (Hussell 1983). Subadults with less than 50% blue plumage are 1 yr old, and subadult females that have greater than 50% blue plumage (about 20% of all subadults) tend to be 1 or 2 yr old (Hussell 1983). Each captured bird was uniquely marked on the wings or tail with different colors of acrylic paint, so that members of a resident pair could be easily distinguished from a distance.
To observe the response of residents toward intruders, we conducted box watches in the NL and NES in 1984 and the NES in 1985. In each population we observed 10 resident pairs where the female was an adult and where at least one member of the pair was individually marked. Watches were 30 min long and were conducted between 0600 and 1200 every 4- 5 days on each of the nest sites throughout the breeding season. An intrusion was recorded when a conspecific entered the territory of the resident pair, defined as an area with a radius of about 15-20 m around the nest site (Robertson and Gibbs 1982, Muldal et al. 1985). For each intrusion we recorded whether the intruder had an adult or subadult plumage and the response of the resident male and female toward the intruder. We classified the behavior of each resident as out of sight (absent, inside nest box), present but no aggressive response, mild response (brief chatter), or strong response (persistent chatter, guarding the nest hole, chasing). Most intruders were not marked with paint, so we could not identify repeated intrusions by the same individual. To minimize the possible lack of independence of those intrusions, we included intrusions in the analysis only if there had not been an intruder of the same color in the previous 5 min. For a given half-hour watch, there were usually only 1 or 2 intrusions by birds of a color that met those criteria; therefore, we considered the response of residents to different intrusions to be independent.