Manipulating carer number versus brood size: complementary but not equivalent ways of quantifying carer effects on offspring.

Experiments designed to quantify the effects of increasing numbers of carers on levels of offspring care are rare in cooperative breeding systems, where offspring are reared by individuals additional to the breeding pair. This paucity might stem from disagreement over the most appropriate manipulations necessary to elucidate these effects. Here, we perform both carer removal and brood enhancement experiments to test the effects of numbers of carers and carer:offspring ratios on provisioning rates in the cooperatively breeding chestnut-crowned babbler (Pomatostomus ruficeps). Removing carers caused linear reductions in overall brood provisioning rates. Further analyses failed to provide evidence that this effect was influenced by territory quality or disruption of group dynamics stemming from the removals. Likewise, adding nestlings to broods caused linear increases in brood provisioning rates, suggesting carers are responsive to increasing offspring demand. However, the 2 experiments did not generate quantitatively equivalent results: Each nestling received more food following brood size manipulation than carer removal, despite comparable carer:offspring ratios in each. Following an at-hatching split-design cross-fostering manipulation to break any links between prehatching maternal effects and posthatching begging patterns, we found that begging intensity increased in larger broods after controlling for metrics of hunger. These findings suggest that manipulation of brood size can, in itself, influence nestling provisioning rates when begging intensity is affected by scramble competition. We highlight that carer number and brood size manipulations are complimentary but not equivalent; adopting both can yield greater overall insight into carer effects in cooperative breeding systems.

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 hypothesized 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 per cent of group members with kin present helped, 100 per cent directed their care towards the most related brood in the social group, and those rearing half/full-sibs helped approximately three 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.