Social complexity can drive vocal complexity: group size influences vocal information in Carolina chickadees.

One hypothesis to explain variation in vocal communication in animal species is that the complexity of the social group influences the group's vocal complexity. This social-complexity hypothesis for communication is also central to recent arguments regarding the origins of human language, but experimental tests of the hypothesis are lacking. This study investigated whether group size, a fundamental component of social complexity, influences the complexity of a call functioning in the social organization of Carolina chickadees, Poecile carolinensis. In unmanipulated field settings, calls of individuals in larger groups had greater complexity (more information) than calls of individuals in smaller groups. In aviary settings manipulating group size, individuals in larger groups used calls with greater complexity than individuals in smaller groups. These results indicate that social complexity can influence communicative complexity in this species.