Researchers have reported that both Tachycineta bicolor (Tree Swallow) and Tachycineta thalassina (Violet-green Swallow) are more aggressive than Sialia sialis (Eastern Bluebird) and often outcompete bluebirds for nest sites. Therefore, we would not have expected that Tree Swallows would be victims of usurpation by Eastern Bluebirds. However, as part of a study on Tree Swallows, we found that although Tree Swallows occupied 68% of the nest boxes during May, there was also evidence of nest loss in Tree Swallows due to usurpation by Eastern Bluebirds. Suspected usurpation by Eastern Bluebirds resulted in the loss of 18% of Tree Swallow nests over 2 years. Almost all usurpation events coincided with the conclusion of the first nesting cycle in Eastern Bluebirds. Our data suggest that usurpation by Eastern Bluebirds is more common than reported in the literature.
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Vol. 23 • No. 4