The Cost of Polygyny in the House Wren Troglodytes aedon

1. 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. 2. In each of three breeding seasons, females who chose mated males (i.e. `secondary' = 2 degrees females) produced significantly fewer fledglings per breeding attempt than females who chose unmated males (monogamous (M degrees) and primary (1 degrees) females). Secondary females were less likely than M degrees /1 degrees 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. Secondary females also produced fewer fledglings in completed nesting attempts, primarily because they lost more offspring to starvation during the first two-thirds of the nestling stage. Nestlings raised by 2 degrees females weighed less than nestlings raised by M degrees /1 degrees females late in the nestling stage. 3. Reduced male aid in feeding young partially explains the lowered reproductive success of 2 degrees females. Whereas males provided substantial aid to M degrees and 1 degrees mates, most 2 degrees females received little or no aid. The few 2 degrees females who did receive substantial male aid produced more fledglings than 2 degrees females without male aid suggesting that the lowered success of 2 degrees females observed on a population-wide basis did not solely result from 2 degrees females being younger or less-experienced breeders. 4. These results allow rejection of several hypotheses for territorial polygyny which assume that mate sharing is not costly to polygynously mating females.