The concept of effective population size (N-e) is used widely by conservation and evolutionary biologists as an indicator of the genetic state of populations, but its precision and relation to the census population size is often uncertain. Extra-pair fertilizations have the potential to bias estimates of N-e when they affect the number of breeders or their estimated reproductive success tallied from social pedigrees. We tested if the occurrence of extra-pair fertilizations influenced estimates of N-e in a resident population of song sparrows Melospiza melodia using four years of detailed behavioural and genetic data. Estimates of N-e based on social and genetic data were nearly identical and averaged c. 65% of the census population size over four years, despite that 28% of 471 independent young were sired outside of social pairs. Variance in male reproductive success also did not differ between estimates based on social and genetic data, indicating that extra-pair mating had little effect on the distribution of reproductive success in our study population. Our results show that the genetic assignment will not always be necessary to estimate N-e precisely.