It has previously been argued that the feeding of nestlings by non-parental birds may simply be an unselected consequence of delayed dispersal in cooperative breeding birds in which individuals follow simple rules such as `feed any begging mouth in my natal territory'. However, in the cooperative breeding Seychelles warblers (Acrocephalus sechellensis), helpers are more likely to help feed full siblings (both parents still alive) than half-siblings (one parent died), and do not help at all when the young are unrelated (both parents replaced). Helpers, helping both full siblings and half-siblings reduced their helping effort (food provisioning and period of helping) significantly when rearing young of lesser relatedness. These behaviours suggest that helping has been selected for in the context of promoting an individual's indirect fitness, and that it is not simply a by-product of `provisioning behaviour'. The mechanism by which kin discrimination in helping is achieved appears to be associative learning; birds more often became helpers at nests belonging to related individuals who fed them (as a parent or a helper) when they were nestlings than at nests of related breeders who had not fed them in the nest.