Preferred Nest Spacing of an Obligate Cavity-Nesting Bird, the Tree Swallow

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Abstract
In order to determine the preferred dispersion of a population of breeding Tree Swallows (Tachycineta bicolor) in Ontario [Canada], 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.