The study was conducted during 1983 and 1984 at the Queen's University Biological Station, near Chaffey's Lock, approximately 50 km north of Kingston, Ontario. Two Tree Swallow populations were studied, both of which have been established for at least 5 yr. These appear to be 2 separate breeding populations, since only 2 out of 50 individuals that have been captured in the last 2 or 3 consecutive years are known to have moved between populations. One study site, the New Land (NL), was about 10 ha in size and consisted of several hay fields and two small ponds. The area was bordered by deciduous forest and was less than 1 km from open water. All nest sites were plywood nest boxes, and most were mounted on aluminum posts. The other study site, the Northeast Sanctuary (NES) of Lake Opinicon, was 10 km from the NL. Nest sites were distributed over approximately 5 ha of open, shallow water and consisted of both nest boxes and natural hollow stumps. The size of both populations increased between 1979 and 1981, but has remained stable since 1982. In May 1982, there were 54 breeding pairs in the NL and 30 pairs in the NES.