We collected genetic and behavioural data on hihi (Notiomysts cincta, an endangered New Zealand bird) after reintroduction to Mokoia Island to assess the effect of extrapair copulation on effective population size (Ne), and investigate the potential for increasing Ne through behavioural management. DNA fingerprinting revealed that 46% of chicks (n=188) resulted from extra-pair paternity, and 82% of broods (n=56) had at least one extra-pair chick. Of the extra-pair young, 34% (n=89) were from unpaired males, and the remainder were from paired males. Variance in reproductive success (VRS) among individuals changed between years, and the relative variance among males and females depended on the sex ratio. VRS increased when measured over longer time scales, the variance in recruits being three times higher than the variance in the number of hatchlings. Extra-pair copulation increased VRS by 150% in 1 year and decreased it by 30% in another year, but this only caused a 4% decrease and 8% increase, respectively, to Ne=N. Although there is potential to manage VRS in this species through behavioural management, a more important factor is adult lifespan, which is the main correlate of lifetime reproductive success as well as the determinant of generation time. The high annual mortality rate in Mokoia hihi (females=64%, males= 52%) has prevented the population from growing, so the key factors limiting N and Ne=N are the same.