Across diverse societies, group members benefit from the cooperative maintenance of a shared territory (a public good). How such public goods are maintained has received extensive interest, yet individual contributions to cooperative territory defence remain poorly understood. Recent theory predicts that, in groups with social hierarchies, privileged individuals will contribute most to competition with rival groups as they benefit most from defence of the territory. Here, we investigated whether dominant individuals contribute more to territory defence in a group-territorial bird in which dominants monopolize within-group reproduction: the white-browed sparrow-weaver, Plocepasser mahali. Using simulated territorial intrusions, we demonstrate that dominants contributed significantly more than subordinates to territory defence. We also found that individual contributions were adjusted according to threat: males of both social classes significantly and similarly increased their contributions to defence in response to a high threat (playback of an unfamiliar pair's duet, rather than that of a neighbouring pair), which was associated with a stronger collective response by the group. Thus, while dominants contributed most as predicted by the asymmetry in benefits, subordinates did increase contributions when they were needed most (in small groups and under greater threat). Contributions by subordinates when needed most also highlights that dominants could still benefit substantially from tolerating the presence of subordinates despite their overall lower contributions. Our results show that public goods can be maintained despite unequal contributions and highlight the potential importance of context-dependent behavioural flexibility in mitigating collective action problems.