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. 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.
Highlights
The level of sustenance received by offspring during development has significant impacts on their survival and subsequent reproductive potential (Lindström 1999)
Groups used in the brood size manipulations provisioned an average of 12.7 ± 6.8 (SD) items/h on control days, a comparable rate to those observed on control days in the carer removal experiment (T31 = −0.29, P = 0.78)
To elucidate whether territory quality contributed significantly to the apparently causal effects of carer numbers on brood provisioning rates, we investigated the relative impacts of control versus experimental numbers of male carers on per capita rates of nestling food acquisition on manipulation days
Summary
The level of sustenance received by offspring during development has significant impacts on their survival and subsequent reproductive potential (Lindström 1999). A meta-analysis of experimental studies confirms putative fathers have additive effects on brood provisioning rates because increases in their contributions are generally met with only partial reductions by their partner (Harrison et al 2009) This meta-analysis revealed that the type of manipulation performed influences rates of food acquisition by the brood. The degree to which offspring benefit from the addition of more carers in cooperative breeding systems has been subject to relatively few experimental tests, and fewer still have clarified whether increasing carer number positively influences the level of sustenance received by offspring This may be in part due to the fact that there is disagreement over the type of manipulation that should be performed to quantify the impact of increasing carer number on offspring success in cooperative breeders (Cockburn 1998), and no study has compared candidate methods
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Schedule a callMore From: Behavioral ecology : official journal of the International Society for Behavioral Ecology
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