Compensatory and additive helper effects in the cooperatively breeding Seychelles warbler (Acrocephalus sechellensis).

O_LIIn cooperatively breeding species, helper aid may affect dominant breeders investment trade-offs between current and future reproduction. By compensating for the care provided by helpers, breeders can reduce the costs of reproduction and improve chances of survival. Also, helper care can be additive to that of dominants, resulting in higher success of the current brood.\nC_LIO_LIHowever, the influence of helpers on offspring care itself may be the by-product of group size and territory quality. Therefore to make conclusive inferences about causation of additive and compensatory care as a result of help per se requires disentangling the impact of helping from other factors determining parental investment.\nC_LIO_LIIn this study, we use 20 years of offspring provisioning data to investigate the effect of helping on breeder and overall offspring provisioning rates in the facultative cooperatively breeding Seychelles warbler (Acrocephalus sechellensis). Our extensive dataset allowed us to effectively control for the effects of living in a larger group and in territories with higher food availability.\nC_LIO_LIWe show compensatory and additive care in response to helper aid. Helpers lightened the provisioning load of the dominant male and female and increased the total provisioning to the nestlings. This was irrespective of group size or territory quality (food availability).\nC_LIO_LIOur results illustrate how multiple benefits of helping behaviour can simultaneously be fundamental to the evolutionary maintenance of cooperative behaviour.\nC_LI