Previous studies thought that at the within-population level, whether a female bird engages in extra-pair (EP) mating depends on how synchronous she is in breeding time with all other females around her, presumably the synchronization might affect the female's opportunities to meet potential EP sires who socially pair with these other females. However, when females or males are choosy about EP partners and mate with one EP individual only, the probability of EP mating may be most influenced by breeding synchrony between the EP partners. In such a case, the ‘individual-level’ synchrony should act to determine EP mating success. We test this idea in a socially monogamous passerine, the ground tit Parus humilis. Fifty-five out of 172 sampled females produced 122 EP offspring, each mating with one EP sire in most cases (92%), usually her intermediately-related kin. As expected, the broader-scale synchrony did not predict the probability of EP paternity but the individual-level did, for females having EP offspring bred more synchronously with their EP than with their nearest neighbors, and females without EP offspring were least synchronous with their nearest neighbors. We argue that this kind of individual-based approaches will shine light on the synchrony-EP mating relationship in birds.