The social environment during adolescence affects adult behaviour in many species, but the functional significance of these effects is not well understood. Zebra finches are highly social colonial breeders but sometimes breed at a distance from others. Early social environment may thus predict adult reproductive competition. We found earlier that males growing up in a group show less courtship and aggression as adults than males reared in pairs. The behaviour of pair-reared males may be beneficial at low densities but detrimental when breeding colonially. We tested this by comparing reproductive performance of pair- and group-reared males under low density (two males and one female) and high density (5 males and 4 females) social environments. Suprisingly, pair-reared males produced more offspring under high densities, whereas group-reared males were more successful under low density. Although this match-mismatch effect supports the idea that male behaviour is adaptively shaped by the early social environment, it is also clear that the influence of adult social conditions is more complex than expected. Reproductive success also appeared to enhance male aggressiveness and reduce male courtship. This contrasts with permanent effects of the adolescent social environment found earlier. The most likely explanation is that behaviour was still plastic in our study, since males were placed into breeding aviaries during early adulthood. Future studies should focus in more detail on how the timing of social experience between early development and first reproduction affects adult behaviour. Also, to understand the functional relevance of the observed effects, reproductive performance should be compared under a range conditions that reflect the changing costs and benefits of sociality and reproductive competition under different densities in the natural environment.