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

Abstract

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. We found no evidence to suggest 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 two 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 pre-hatching maternal effects and post-hatching 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