Carer provisioning rules in an obligate cooperative breeder: prey type, size and delivery rate

Providing food to developing offspring is beneficial for offspring but costly for carers. Understanding patterns of provisioning thus yields important insights into how selection shapes (allo-) parental care strategies. Broadly, offspring development will be influenced by three components of provisioning (prey type, size and delivery rate). However, all three variables are rarely considered simultaneously, leading to suggestions that the results of many studies are misleading. Additionally, few studies have examined the provisioning strategies of breeders and non-breeding helpers in obligate cooperative breeders, wherein reproduction without help is typically unsuccessful. We investigated these components of provisioning in obligately cooperative chestnut-crowned babblers (Pomatostomus ruficeps). Prey type was associated with size, and delivery rate was the best predictor of the overall amount of food provided by carers. As broods aged, breeders and helpers similarly modified the relative proportion of different prey provided and increased both prey size and delivery rate. Breeding females contributed less prey than male breeders and adult helpers, and were the only carers to load-lighten by reducing their provisioning rates in the presence of additional carers. While our results suggest that breeders and helpers follow broadly comparable provisioning rules, they are also consistent with the idea that, in obligately cooperative species, breeding females benefit more from conserving resources for future reproduction than do helpers which have a low probability of breeding independently.