Why do individuals live in groups? Group living occurs across a variety of organisms, from bees to humans, and has facilitated the evolution of behaviors including division of labor and sophisticated communication. However, living in a group is costly: group members may be forced into direct competition with one another, or may have to give up other potentially profitable opportunities in order to join the group. How can we predict which conditions will favor the evolution of group living ("sociality") in spite of the costs? Here, I will explore the insights that bird societies can contribute to understanding the evolutionary pressures that lead to group living. First, I'll explain a life history-based approach that allows us to understand why cooperative breeding (a form of group living) can still occur in species with high rates of extra-pair paternity, which reduces the relatedness between a helper and the recipient of help. Second, I'll show how living in a group with kin influences competition between males over extra-pair paternity, and how the scale at which individuals compete for breeding positions (local versus global) mediates this competition. Third, I'll investigate why many species change their social behavior with the seasons, and discuss how understanding this seasonal sociality can help illuminate the evolution of group living more generally.

Caitlin Stern, PhD and Omidyar Fellow, Santa Fe Institute, evolutionary biologist, 

www.cstern.org