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

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.Funding provided by: American Philosophical SocietyCrossref Funder Registry ID: http://dx.doi.org/10.13039/100001461Award Number: Funding provided by: National Science FoundationCrossref Funder Registry ID: http://dx.doi.org/10.13039/100000001Award Number: IOS-1755279Funding provided by: National Science FoundationCrossref Funder Registry ID: http://dx.doi.org/10.13039/100000001Award Number: IOS-184543Funding provided by: High Meadows Environmental Institute, Princeton UniversityCrossref Funder Registry ID: http://dx.doi.org/10.13039/100020601Award Number: Funding provided by: Office of Undergraduate Research, Princeton University*Crossref Funder Registry ID: Award Number: Funding provided by: Department of Ecology and Evolutionary Biology, Princeton University*Crossref Funder Registry ID: Award Number: Funding provided by: Program in Latin American Studies, Princeton University*Crossref Funder Registry ID: Award Number: Funding provided by: National Science FoundationCrossref Funder Registry ID: http://dx.doi.org/10.13039/100000001Award Number: Graduate Research Fellowship ProgramFunding provided by: Princeton Institute for International and Regional StudiesCrossref Funder Registry ID: http://dx.doi.org/10.13039/100008141Award Number