Research on sexually selected male traits has intensified over the last two decades and there are now many species of birds for which male traits associated with extra-pair mating success have been identified. Some of the more commonly studied species have been examined in multiple populations; however, there is often little consistency in results between studies. In paired analyses of Tree Swallows breeding in Wisconsin, we found that successful extra-pair sires were more experienced breeders, heavier, had fewer lice holes in their feathers, had longer wings and tended to be in better condition than the male they cuckolded. Our results are similar to those from a population of Tree Swallows studied in Ontario but differ from a population studied in British Columbia. Such variation among populations may arise if environmental heterogeneity influences the information content, reliability, or importance of particular male traits as signals of male quality. Taking these factors into consideration will help us understand how selection on male traits varies between populations and the role of extra-pair mating in sexual selection.