Chapters 1 and 2 focus on two aspects of trilled song that are likely physically challenge to produce: low vocal deviation and high consistency. Chapter 1 (in review at Behavioral Ecology) describes a playback study to territorial male house wrens showing that males do not respond differently to songs with different trill performance and consistency characteristics. Chapter 2 examines whether song traits correlate with male quality measures, with social pairing success, with extra-pair mating success, or with annual reproductive success. Again, I found no evidence that these song parameters are important. I conclude that house wrens do not use these song components as signals of male quality. In Chapters 3 and 4, I examine post-copulatory sexual selection in house wrens. Chapter 3 (accepted at the Journal of Ornithology) examines correlations between male quality measures and sperm morphology; we also show that sperm morphology is consistent between years, despite testicular regression and regrowth. In Chapter 4, we ask whether variation in sperm morphology is related to extra-pair paternity success, and find that it is not. Chapter 5 (accepted at Ethology) takes a more mechanistic perspective, and asks whether high circulating testosterone is necessary for aggressive behaviors in house wrens. I found no evidence to suggest that this is the case.