In socially monogamous species, male fertilization-related behaviors are expected to be tuned to the fertile cycle of their social female. How a male adjusts his fertilization-related behaviors to his social mate’s fertile period is relatively well understood. However, the influence of the social mate’s fertility stage on a male’s extrapair siring success remains poorly studied. In this study, we tested 2 alternative hypotheses about how a male’s extrapair siring success may vary in relation to his social mate’s fertile cycle. The “trade-off avoidance” hypothesis posits that males engage in extrapair fertilization-related behaviors when their social female is not fertile to avoid a potential trade-off between within-pair and extrapair reproduction. Alternatively, the “pair synchrony spillover” hypothesis postulates that males are more likely to sire extrapair offspring when their social mate is fertile because male investment in fertilization-related behaviors during their social mate’s fertile period aimed at increasing within-pair siring success will have positive “spillover” effects on their extrapair siring success. To assess the support for each of these 2 hypotheses, we studied a population of great tits (Parus major) for 4 consecutive years in 12 nest-box plots and quantified each male’s extrapair siring success before, during, and after the fertile period of his social mate. After correcting for variation in the availability of fertile females, males gained more extrapair paternity during the fertile period of their social mate, supporting the “pair synchrony spillover” hypothesis.