Summary
We present data on two methods of sexing adult Tree Swallows before they develop a brood patch or cloacal protuberance. Based on measurements of 219 adults, 100% of the birds with a brown forehead (N = 85) were females. Among the 140 known adult females examined, 61% had a brown forehead. The sex of adults could also be classified with 95% accuracy based on a flattened and straightened wing length of 113 mm (females) or >122 mm (males). Almost half of the adult Tree Swallows caught early in the season (before egg-laying) could be sexed using a combination of these criteria.
Methodology
The Tree Swallows in this study were captured in 1985 and 1986 from two populations that breed in grids of nest boxes at the Queen's University Biological Station, Chaffey's Lock, 50 km north of Kingston, Ontario. The New Land (NL) population consists of about 70 nest boxes distributed throughout several hayfields. The Northeast Sanctuary (NES) population has about 40 nest boxes over open, shallow water. Beginning in early April and continuing through the breeding season, Tree Swallows were captured using mist nets and nest box traps (Stutchbury and Robertson 1986). We avoided capturing birds in their nest boxes during egg-laying, since they have a tendency to abandon their nests at this time (Burtt and Tuttle 1983, Cohen 1985). Each bird was banded with a Canadian Wildlife Service numbered band. Birds with less than 90% iridescent blue-green plumage over the upper parts of the body (subadult plumage) were classified as females (Hussell 1983). Once breeding had begun, birds with adult plumage were sexed by the presence of a brood patch (female) or cloacal protuberance (male). In many cases, the sex of resident birds was later confirmed by observations of copulations, and identifying the mate of known females by observing territorial and parental behavior.