Data from: Joint care can outweigh costs of nonkin competition in communal breeders

Competition between offspring can greatly influence offspring fitness and
parental investment decisions, especially in communal breeders where
unrelated competitors have less incentive to concede resources. Given the
potential for escalated conflict, it remains unclear what mechanisms
facilitate the evolution of communal breeding among unrelated females.
Resolving this question requires simultaneous consideration of offspring
in noncommunal and communal nurseries, but such comparisons are missing.
In the Seychelles warbler Acrocephalus sechellensis, we compare nestling
pairs from communal nests (2 mothers) and noncommunal nests (1 mother)
with singleton nestlings. Our results indicate that increased provisioning
rate can act as a mechanism to mitigate the costs of offspring rivalry
among nonkin. Increased provisioning in communal broods, as a consequence
of having 2 female parents, mitigates any elevated costs of offspring
rivalry among nonkin: per-capita provisioning and survival was equal in
communal broods and singletons, but lower in noncommunal broods.
Individual offspring costs were also more divergent in noncommunal broods,
likely because resource limitation exacerbates differences in competitive
ability between nestlings. It is typically assumed that offspring rivalry
among nonkin 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 providing a mechanism by
which communal breeders can avoid these costs.