Todd Freeberg, Jeffrey Lucas, and Indrikis Krams in American Scientist:
If you live in North America, Europe or Asia near a forest, suburban open woodlands or even an urban city park, chances are you have heard a member of the avian family Paridae—the chickadees, tits and titmice. Birds use calls to communicate with their flockmates, and most parids share a unique call system, the chick-a-dee call. The call has multiple notes that are arranged in diverse ways. The resulting variation is extraordinary: The chick-a-dee call is one of the most complex signaling systems documented in nonhuman animal species.
Much research on the chick-a-dee call has considered Carolina chickadees,Poecile carolinensis, a species common in the southeastern United States. We focus on this species here, but we also compare findings from other parids. We discuss how the production and reception of these calls may be shaped over individual development, and also how ecological and evolutionary processes may affect call use. Finally, we raise some key questions that must be addressed to unravel some of the complexities of this intriguing signaling system. Increased understanding of the processes and pressures affectingchick-a-dee calls might tell us something important about what drives signaling complexity in animals, and it may also help us understand the evolution of that most complex vocal system, human language.