Experimental evidence for fully additive care among male carers in the cooperatively breeding chestnut-crowned babbler

Although theory developed to understand carer response rules in cooperative breeders typically predicts partial compensation, where additional investment by one carer is optimally met by incomplete reductions by the other, fully additive care is a viable alternative under particular conditions. Primary among these conditions is an opportunity for both existing and additional carers to gain comparable fitness from contributing to rearing offspring. That, in a number of cooperative birds, at least one parent often maintains its level of contribution to offspring rearing independent of carer numbers is supportive, but experimental evidence is lacking. Here, in naturally occurring groups of the cooperatively breeding chestnut-crowned babbler, Pomatostomus ruficeps, we found that provisioning rates of male carers were insensitive to the number of other males present; this resulted in an increase in total brood and per capita nestling provisioning rates across the range of total carer numbers tested (i.e. two - seven). Further, remaining male carers failed to change their provisioning rates following the temporary removal of one to three other males for up to 36 h, leading to significant decreases in total brood and per capita nestling provisioning. We found no obvious evidence to suggest that carer removals were otherwise disruptive and confounded the opportunity for remaining carers to respond. Our results confirm the existence of strongly additive care in cooperative breeders, and corroborate recent theory predicting that such response rules will arise when all carers in a group have the potential to contribute similarly to offspring success.