Effects of Predation and Competitor Interference on Nesting Success of House Wrens and Tree Swallows

Abstract

I examined the relationships among brood survival in House Wrens (Trog- lodytes aedon) and Tree Swallows (Tachycineta bicolor) and rates of nest-box use, species interference, and nest predation. Tree Swallows nested in boxes in one of three woodlands occupied by House Wrens. Over a 4-year period, clutch mortality rates in swallows were significantly higher than those in wrens, but wrens on swallow-free plots had lower failure rates than wrens that coexisted with swallows. Though conspecifics interfered at 9 of 99 (9%) wren nests, predation was the major cause of wren nest failure, accounting for 70% of 27 unsuccessful attempts. Increased nest failure in wrens was associated with increased rates of box use. In contrast, clutch mortality in Tree Swallows was related to nest interference by wrens. Of 29 swallow nests, 13 (45%) showed signs of interference by wrens, and eight of these 13 (62%) were initiated in boxes containing empty nests built earlier by wrens. House Wrens are interference competitors because they exclude swallows from boxes by destroying swallow nests. Destroying nests of other species has advantages for wrens if high concentrations of empty nests, including dummy nests, inhibits search efficiency of predators. An experimental approach is recommended for testing the hypothesis that House Wrens build dummy nests and destroy heterospecific nests because empty nests deceive predators.