Data from: Joint care can outweigh costs of non-kin competition in communal breeders

Competition between offspring can greatly influence offspring fitness and parental investment decisions, especially in communal breeders where the offspring that are less related to each other have less incentive to concede resources. Given the potential for escalated conflict, it remains unclear what mechanisms facilitate the evolution of communal breeding among females. Resolving this question requires simultaneous consideration of offspring in non-communal and communal nurseries, but such comparisons are largely missing. In the Seychelles warbler Acrocephalus sechellensis, we compare nestling pairs from communal nests (two offspring, two mothers) and non-communal nests (two offspring, one mother) with singleton nestlings. Our results indicate that increased provisioning rate in communal broods can act as a mechanism to mitigate the costs of offspring rivalry among non-siblings. Increased provisioning as a consequence of having two female parents mitigates any elevated costs of offspring rivalry among non-kin: per-capita provisioning and survival was equal in communal broods and singletons, but lower in non-communal broods. Individual offspring costs were also more divergent in non-communal broods, likely because resource limitation exacerbates differences in competitive ability between nestlings. It is typically assumed that offspring rivalry among non-siblings will be more costly because offspring are not driven by kin selection to concede resources to their competitors. Our findings are correlational and require further corroboration, but may help explain the evolutionary maintenance of communal breeding by identifying a mechanism by which communal breeders can avoid escalated costs of sibling rivalry among non-kin.