The Cost of Polygyny in the House Wren Troglodytes aedon

Our objective was to determine why territorial polygyny occurs in the house wren Troglodytes aedon, a small, insectivorous, cavity-nesting songbird. In this paper, we present evidence that choosing a mated male in our Wyoming (USA) study population is inherently costly to female fitness. In each of three breeding seasons, females who chose mated males (i.e. «secondary» = 2° females) produced significantly fewer fledglings per breeding attempt than females who chose unmated males (monogamous (M°) and primary (1°) females). Secondary females were less likely than M°/1° females to complete nesting attempts, primarily because they lost more broods to starvation, and had more clutches and broods destroyed by conspecifics intruding onto territories