Data from: Kin selection, not group augmentation, predicts helping in an obligate cooperatively breeding bird

Kin selection theory has been the central model for understanding the
evolution of cooperative breeding, where non-breeders help bear the cost
of rearing young. Recently the dominance of this idea has been questioned;
particularly in obligate cooperative breeders where breeding without help
is uncommon and seldom successful. In such systems, the direct benefits
gained through augmenting current group size have been hypothesised to
provide a tractable alternative (or addition) to kin selection. However,
clear empirical tests of the opposing predictions are lacking. Here, we
provide convincing evidence to suggest that kin selection and not group
augmentation accounts for decisions of whether, where and how often to
help in an obligate cooperative breeder, the chestnut-crowned babbler
(Pomatostomus ruficeps). We found no evidence that group members base
helping decisions on the size of breeding units available in their social
group, despite both correlational and experimental data showing
substantial variation in the degree to which helpers affect productivity
in units of different size. By contrast, 98% of group members with kin
present helped, 100% directed their care towards the most related brood in
the social group and those rearing half/full-sibs helped approximately 3
times harder than those rearing less/non-related broods. We conclude that
kin selection plays a central role in the maintenance of cooperative
breeding in this species, despite the apparent importance of living in
large groups.