Effect of Egg Covering and Habitat on Nest Destruction by House Wrens

Abstract

Birds nesting near House Wrens (Troglodytes aedon) risk having their eggs, nestlings, and nests destroyed. Damage by wrens may be reduced in Black-capped Chickadees (Parus atricapillus), Tufted Titmice (P. bicolor), and other parids by concealing eggs under nest material during the laying period, and in sympatric cavity-nesting species by nesting in different habitats from wrens. To test if eggs were protected by covering, prelaying wrens were challenged for 1 day with a set of two boxes placed 1 m from their nest, one with two artificial eggs (miniature marshmallows) lightly covered under fur, the other with two artificial eggs in an open cup. Results varied with stage of nest-building; in 41 trials where both exposed eggs were removed, covered eggs remained in only 4 of 15 (27%) trials near early nests containing few sticks, but in 17 of 26 (65%) trials near more advanced nests. To assess effects of nest site, a box with a cup nest was placed in each of three habitats 10 or 20 m from 29 wren nests. After 1 day of habituation, two artificial eggs were placed in each nest and left exposed for 1 day. Boxes in woodland interiors were less likely than boxes in fields and along edges to be visited by wrens at least once over 2 days (66 vs. 97% visited) and were less likely to have eggs removed (10 vs. 83% removed). Competitors for nesting cavities also may escape attacks by wrens through differences in breeding period, active defense of territories or nests, or renesting.