Variation in life-history traits has long captivated ecologists and evolutionary biologists. Early contributors identified latitudinal clines in life-history traits and proposed ecological hypotheses to explain this variation. One ecological hypothesis proposed to explain geographic variation in extra-pair paternity (EPP) is the breeding synchrony hypothesis. Under this hypothesis, synchronously breeding females will be better able to assess the quality of potential mates when making mating decisions. The prediction in this hypothesis is that synchrony increases towards the poles because of shorter breeding seasons; rates of EPP are therefore expected to increase towards higher latitudes. Simultaneously, recent comparative work found that most diversification in avian EPP occurred early in the evolutionary history of birds, with most variation found between Families and Orders. In my dissertation I explore these two perspectives by examining interspecific variation in genetic mating system in the swallow genus Tachycineta. I obtained EPP data using microsatellite markers for five species of Tachycineta swallows ranging from Tierra del Fuego to British Columbia. Tachycineta swallows exhibit substantial variation in EPP, with 12 to 89% of nests having extra-pair young. A notable example of this variation is found between the sister taxa T. leucorrhoa and T. meyeni, with 78% and 12% of nests with extra-pair young respectively. My results indicate that breeding synchrony is not a strong predictor of EPP rates across species and latitudes. Additionally, I provide a detailed analysis of fitness benefits of EPP for a south-temperate species, T. leucorrhoa. I found that T. leucorrhoa nests with extra-pair young fledge more offspring compared to those with all within-pair young. However, I did not find support for a link between this fitness advantage and the level of heterozygosity as proposed by theory. Work on Tachycineta helps redress the paucity of information on tropical and south-temperate species and an underrepresentation of closely related taxa that characterized previous studies. It also reminds us of the complexity of relationships among life-history traits and their environmental influences, forcing us to consider more than one hypothesis and causal path in explaining hemisphere-wide patterns in life histories.