Summary
We examined 10 characteristics of natural cavities and their influence on reproductive success of Tree Swallows (Tachycineta bicolor) nesting in dead trees in beaver ponds. Large ranges were found for entrance height and area, cavity volume, and nearness to shore of nest sites. Other characteristics were less variable: 46% of cavities were less than 2 m above the pond surface, and 48% had entrance widths of 4-5 cm. Tree Swallow nest sites were uniformly dispersed in the ponds. Two cavity characteristics, cavity height and floor area, influenced reproductive success. Lower nest sites were more frequently preyed on and females laid larger clutches in cavities with a large floor area. Five species larger than Tree Swallows used cavities during the study. Girth of the snag at the base and at the cavity, entrance width, and cavity volume were significantly greater at sites used by these species than those used by Tree Swallows. Nest sites suitable for breeding did not appear to be limiting to Tree Swallows, because characteristics of unused cavities did not differ from those used by Tree Swallows and other species. Intraspecific territoriality was likely responsible for the large number of unused cavities in our populations. Other factors influencing cavity availability in our sites include interspecific competition, predation, snag fall, and continuing woodpecker excavation
Methodology
This study was conducted in 1986 and 1987 near the Queen's University Biological Station, 50 km north of Kingston, Ontario, Canada. The study sites, Allan's Pond (AP) and Osprey Marsh (OM), are woodlands flooded by beaver dams. The two sites are 8 km apart. Hundreds of snags (dead trees) remain standing in water at each site. These snags are excavated by woodpeckers for nesting and roosting and the excavations are subsequently occupied by Tree Swallows and other species. AP is 4 ha with snags distributed evenly throughout the pond. OM is 11 ha with most snags clumped in the south-central part of the pond. The northern half of OM is covered in cattail beds. Water levels in both ponds fluctuate substantially each year due to dam quality and rainfall (mean depths = 0.5-1.5 m). The catchment about each pond is deciduous woodland consisting primarily of maple (Acer spp.), oak (Quercus spp.), poplar (Populus spp.), and basswood (Tilia spp.). The standing snags at each site were most likely of these species.