Prey Size Selection by Tree Swallows

Diets were determined for Tree Swallows (Tachycineta bicolor) that collected flying insects for nestlings and foraged at two locations, 3.25 km apart, where food abundance differed. At both locations, more than 99% of prey in the diets were 10 mm or less in length and at least 95% of the diets consisted of insects from the orders Diptera and Homoptera. Regardless of location, Tree Swallows were selective in the sizes of prey that they delivered to their young. They chose more valuable prey but did not completely reject less valuable items, and the diet was closer to the optimum at the location where food abundance was greatest.