Behaviour 2019
Environmental unpredictability maintains variation in group size in a cooperatively breeding bird
Christina Riehl. Princeton University, Princeton, NJ, United States

Group size varies in nearly all social animals, but explaining the range of group sizes seen in nature remains a challenge. Does this variation represent sub-optimal compromises from an optimum, or do tradeoffs in the costs and benefits of sociality maintain a range of group sizes? In this talk, I will present data from greater anis (Crotophaga major) that support the latter hypothesis. Anis in the study population nest in communal groups containing two or three breeding pairs and 0-2 nonbreeding helpers. Larger groups have lower nest predation (due to cooperative nest defense) but suffer higher nestling starvation (due to intra-clutch competition). Long-term data show that the relative magnitude of these costs and benefits depends on climate: in wet years, per capita reproductive success is higher in large groups than in small groups, whereas the opposite is true in dry years. Over the 12-year study period, per capita reproductive success is approximately equal in small and large groups. These results suggest that variation in climate may partly drive variation in group size, and that a full understanding of the selective pressures shaping group size may require long-term data.