Understanding inter- and intra-specific variation in mate compatibility and reproductive success can offer insight into the factors driving sexual selection, behavioral dynamics, and isolating mechanisms across natural populations. This information is particularly relevant when trying to understand the patterns that shape the causes and outcomes of hybridization in natural systems. We evaluated mating patterns and male reproductive success in a hybrid zone between the Saltmarsh (Ammodramus caudacutus) and Nelson's sparrow (A. nelsoni). Specifically, we investigated variation in male reproductive success between pure Saltmarsh and pure Nelson's sparrows and between pure and hybrid males, while testing for assortative vs. random mating. DNA samples were collected from adults (n = 342) and nestlings (n = 348), and paternity analyses and assignment of individuals to pure and hybrid classes were conducted using 11 microsatellite loci. We documented high promiscuity and reproductive skew in Saltmarsh and Nelson's sparrow males, with greater skew in Saltmarsh Sparrows. Across Nelson's Sparrow and hybrid individuals, we documented a significant correlation between reproductive success and genetic heterozygosity. F1–F2 hybrid males exhibited significantly lower reproductive success (number of offspring sired) compared with pure individuals, and Saltmarsh and Nelson's sparrows exhibited strong patterns of assortative mating. We conclude that differences in reproductive success among pure and hybrid individuals, coupled with strong assortative mating, are shaping hybrid zone dynamics in this system and may be important for maintaining species boundaries.