We investigated whether male parental assistance, reproductive output, and overwinter survival differed for female House Wrens (Troglodytes aedon) that did and did not have to share their mate's territories with a later-settling female during the nestling stage of breeding. During first breeding attempts of the season, when mate sharing occurred, primary females in polygynous trios and females from monogamous pairs fledged equal numbers of offspring and fledged offspring of similar mass. Rates at which males fed broods of primary females were not significantly different than rates at which males fed broods of monogamous females. Primary females did not make significantly more trips to nests to feed offspring than did monogamous females. In the first year of study, primary females were more likely than monogamous females to attempt a second brood. In the second year of the study, however, primary females tended to be less likely to attempt a second brood, took longer to start such broods, and tended to be less successful in fledging offspring from second broods. This suggests that in some years there may be a delayed cost of mate sharing. Females with primary and monogamous status for first breeding attempts in a particular year were equally likely to return to the study area the next year. Overall, our results indicate that mate sharing does not appear to affect the reproductive output of first mated females in our House Wren population in the first breeding attempts of the season, but may affect output in second attempts (and hence possibly annual and lifetime success), at least in some years.