Variation in mating success among individuals is the basis for sexual selection and the evolution of elaborate secondary sexual traits. In socially monogamous species, variation in mating success is generally thought to be small, but a skewed adult sex ratio, differences in female fecundity, and extrapair fertilizations that arise from matings outside the social pair bond can increase variance in reproductive success. We investigated how these factors generate the opportunity for sexual selection in the socially monogamous White-crowned Sparrow (Zonotrichia leucophrys pugetensis). We found a 38% increase in the standardized actual variance in reproductive success compared to the apparent standardized variance of males because of the contribution of extrapair fertilizations to total reproductive success. However, partitioning variance into within-pair and extrapair components showed that the majority of variance in male reproductive success was attributable to within-pair success and a skewed adult sex ratio. Finally, reproductive success increased significantly with the number of mates in males but not in females, which suggests a stronger potential for sexual selection in males than in females in this population. Our results confirm that social mating success may increase the variance in reproductive success more than extrapair fertilizations in some monogamous species. Thus, the evolution of sexual ornaments may be influenced equally or more by the number of available mates and their fecundity than by extrapair matings.