Effects of brood and group size on nestling provisioning and resource allocation in a communal bird

In social groups, receiving assistance from other caregivers can help parents conserve energy and raise more offspring. But what happens when additional caregivers also produce offspring? Do the benefits of receiving extra help exceed the costs of caring for extra young in a shared brood? We show that, in a communally breeding bird, greater anis, costs exceed the benefits: larger broods of offspring experience increased competition that significantly affects resource allocation, despite the presence of additional caregivers. Resource limitations, either due to environmental conditions or constraints on parental provisioning effort, can drive intense competition among offspring. In communal groups, resource availability may increase if parents receive assistance from other group members; however, if those caregivers also produce young, offspring demand may increase at the same time. It is possible, therefore, that the costs of intrabrood competition in large broods may outweigh the benefits of provisioning from additional caregivers. We tested the relationships between group size, brood size, and provisioning rates in the greater ani (Crotophaga major), a communally nesting cuckoo in which multiple breeding pairs and nonreproductive helpers cooperatively raise a shared brood. Crucially, brood and group size can vary independently in this species, allowing us to test changes in each variable separately. Using video footage of 2255 prey deliveries across 10 nests, we found that an increase in the number of adult caregivers within a group did not sufficiently offset a corresponding increase in the number of dependent young within a brood: prey availability per average nestling decreased with brood size, regardless of group size. In larger broods, last-hatched nestlings received significantly less prey than their broodmates, in part due to greater hatching asynchrony that exacerbated competitive asymmetries and facilitated inequality in food allocation. Our results indicate that last-hatched ani nestlings suffer a double cost in large broods: they must compete with more nestmates, and suffer disproportionately from asynchronous hatching. These costs may contribute to increased parent-offspring conflict and may constrain group size in communal breeders.

Automated summary

In social groups, receiving assistance from other caregivers can help parents conserve energy and raise more offspring. However, the costs of caring for extra young in a shared brood may exceed the benefits when additional caregivers also produce offspring. Our study on greater anis, a communally breeding bird, found that larger broods of offspring experience increased competition that significantly affects resource allocation, despite the presence of additional caregivers. These findings suggest that the costs of intrabrood competition in large broods may outweigh the benefits of provisioning from additional caregivers. Rating: 7/10.