Summary
In order to determine the preferred dispersion of a population of breeding Tree Swallows (Tachycineta bicolor) in Ontario, we set up an abundant supply of nest boxes with a variety of distances between them. The 72 boxes were arranged in 12 equidistantly spaced spirals. Within a spiral, the distance between boxes was much smaller than between spirals. Over five breeding seasons, we observed the order and positions in which swallows settled in the spirals. Pairs of swallows usually settled in empty spirals before settling in spirals occupied by conspecifics, but they did not avoid nesting in spirals occupied by Eastern Blue-birds (Sialia sialis). Swallows did not show any spacing preferences when their nearest neighbors were in different spirals, and were therefore more than 36 m away. Within spirals, however, swallows nested as far as possible from each other when their nests were less than 14 days apart. Swallow nests in the same spiral also tended to be spaced out temporally. We conclude that, over the range of distances within a spiral, Tree Swallows prefer to space their nests as far from conspecifics as possible. The observed spacing pattern probably arises from territorial behavior that is directed toward defense of a nest site from intruders.
Methodology
The study was done at the Queen's University Biological Station, Chaffey's Locks, Ontario, during the summers of 1977 to 1981, inclusive. The study site consisted of three adjacent hayfields of different sizes but of similar vegetation. Nest boxes mounted on 1.75-m high poles were erected in a grid of spirals, each spiral consisting of six boxes placed 1, 2, 4, 8, and 16 m from a central box (Fig. 1). This arrangement provided a variety of distances between boxes, both within a single spiral and between spirals. The entrance holes on all the boxes were oriented in the same direction so this did not affect their desirability