Using DNA fingerprinting we studied Henderson Reed-Warblers (Acrocephalus vaughani taiti), which are confined to Henderson Island in the central South Pacific. During the single study season, the birds had a well-defined nesting period from late August to early January. About one-third of nesting groups comprised three, not two adults. The members of trios, which could include two males and one female, or one male and two females, were unrelated. However, all members of trios contributed to incubation and/or feeding the young, whether or not they were parents of the chicks. The output of young per adult was slightly but not significantly higher in pairs than trios. We suggest a compensating advantage for members of trios: in the stable island habitat, young birds may be more readily able to secure a nesting territory when belonging to a trio than when in a pair.