The Complex Call of the Carolina Chickadee
What can the chick-a-dee call teach us about communication and language?
Decades ago the Dutch ethologist Niko Tinbergen described four different “why” questions researchers could ask in trying to understand the behavior they observed in animals. Two of the questions entail proximate approaches that focus on the individual. One of these proximate approaches includes mechanistic questions—what is the neural and physiological basis of the behavior, and what stimuli in the environment elicit behavior? The other proximate approach covers developmental questions—what roles do growth and experience play in shaping and constraining behavior over an individual’s lifetime? The final two questions are ultimate approaches with a population- or species-level focus. These are ecological or functional questions about the adaptiveness of the behavior—what problem might it have evolved in response to?—and they pose phylogenetic or deep-evolutionary questions—how might common ancestry shape and constrain behavior over the existence of a clade? We can use these approaches to help understand the chick-a-dee call.
At a proximate level of analysis, we know that certain environmental stimuli or motivational influences generate variation in calls. In addition, the complexity of social groups in Carolina chickadees can drive complexity in the note composition of calls. In a 2006 study by Freeberg, chickadees placed into large captive flocks used calls with greater information content compared to chickadees placed into small captive flocks, suggesting that the diversity of messages is greater in more complex social groups. These experimental changes to the social groups of chickadees must have generated neural and physiological changes in the individuals in the study, yet we know relatively little about this aspect of the call. Sturdy’s laboratory has carried out a number of exciting studies related to the perception and discrimination of calls in individuals. Female black-capped chickadees reared in isolation fail to develop the ability to perceive relative pitch of males’ songs. However, we know relatively little about the ontogeny of call variation in young parids interacting with parents and, later, with nonrelated adults in their social groups. More work on proximate questions related to call variation is needed.
At an ultimate level of analysis, we can infer that the call is homologous across many different parid species, suggesting a fundamentally comparable call system in common ancestors to today’s chickadees, tits and titmice. We know a fair amount about call variation in a few species, but the calls of most parid species have been little studied, let alone the question of whether call variation corresponds to different environmental or behavioral contexts. As a result, we cannot yet answer many fairly basic questions about the evolution of call variation. At the functional level, we can infer that the call is adaptive in bringing about social cohesion in parid species, because variation in the call can recruit, alarm or potentially signal movement for members of both conspecific and heterospecific flocks. Whether variation in signaling with the call is related to differences in survival or reproduction is an open question. Nonetheless, a number of hypotheses have been proposed to explain the adaptive significance of call variation in parids.
First, the complexity of the social group might influence vocal complexity. This argument is known as the social complexity hypothesis for communicative complexity, and it is supported by findings from a range of mammals, birds and nonavian reptiles, and from auditory, chemical and visual modalities. For the chick-a-dee call, the social complexity hypothesis predicts that populations in which individuals occur in larger groups or in groups with greater network complexity will have more complex calling behavior than populations in which individuals occur in smaller groups or in groups with little network complexity. If future research supports this hypothesis, we will be able to infer that social pressures that arise from interacting with the same individuals over time, in both competitive and cooperative contexts, require a flexible and diverse repertoire of signals. If the complexity of an individual’s social group impacts the diversity of vocal signals used in social interaction, that social group can be seen as both a context for vocal development and a potential selective pressure that could impact vocal behavior.
Selection for increased signaling complexity in parids may also come from other species in mixed-species flocks. For example, Mark Nolen and Lucas found in a 2009 study that both white-breasted nuthatches (Sitta carolinensis) and tufted titmice interact vocally with Carolina chickadees when mobbing predators. The primary vocal signal used by chickadees under these conditions is the chick-a-dee call. Moreover, Chris Templeton and Erick Greene of the University of Montana in 2007 suggested that nuthatches can decode information about predation risk from calls, and recently Stacia Hetrick and Kathryn Sieving of the University of Florida found that chickadees can decode information about predation risk from the chick-a-dee calls of tufted titmice. These findings show that a complex call provides relatively fine-scale information about predation risk to conspecifics and heterospecifics. Both types of association have fitness consequences. The complexity of conspecific and mixed-species flocks may therefore drive the diversity and complexity of vocal signaling systems.
Another hypothesis proposed to explain call complexity is the predation pressure hypothesis, which has support from a number of studies in primate species. It predicts that populations facing intense predation pressure or a variety of predator types should have more complex calling behavior than populations facing relatively light predator pressure. This hypothesis, then, would predict that parid populations or species that face a large number of different predators have a more complex call than parid populations or species that occur in areas with few predators. One more hypothesis to consider for call complexity relates to the physical environment in which individuals live. Parid populations or species living in complex physical environments, such as those containing a mix of open, closed and edge habitat, may require more complex calls to communicate effectively, compared to populations or species living in relatively simple physical habitats, such as exclusively coniferous forests. These three hypotheses (and there are others) may each explain the complexity and variation in chick-a-dee calls that we see. Perhaps our biggest need in answering this question is for large comparative data sets from multiple populations or multiple species, with which to test the various hypotheses.