The Complex Call of the Carolina Chickadee
What can the chick-a-dee call teach us about communication and language?
Complexities upon Complexities
We have discussed sociality in parids in light of the benefits of grouping, but we would be remiss if we did not point out that grouping also brings costs. Foraging in a group reduces energetic costs—individuals have more time to find and process food because they can spend less time detecting predators. But flocking also results in increased competition for resources and may generate higher stress levels. It may also increase transmission of and reduce resistance to parasites and pathogens. More work on the costs of grouping in parids should shed considerable light on the pressures individuals and their signaling systems face in complex social groups.
The Paridae family seems ideal for testing hypotheses for communicative complexity. As Jan Ekman of Uppsala Universitet pointed out in a 1989 study, it has considerable variation across species in key social dimensions such as group size, presence and number of heterospecifics in mixed-species flocks, and presence or absence of winter territories. For example, flocks in great tits (Parus major) are reported to range from 2 to roughly 50 individuals (see Figure 9). It is hard to determine flock size in this species, however, because great tits do not have a stable flock structure over time (individuals often move in and out of groups) or space (their flocks, unlike those of many other parids, are not territorial). Recent advances in assessing social networks in animal groups should prove important to determining social complexity in this species. We believe great tits could be a key species for testing functional hypotheses regarding call complexity.
Does the variation in social complexity we have been describing here explain variation in the structure and use of chick-a-dee calls? This straightforward question, like the questions raised by other hypotheses, remains unanswered simply because social and vocal behavioral data are needed for a greater number of parids than have been studied to date. For example, we know very little about the vocal behavior and social structure of African parids in the species-rich Melaniparus group, or of South and East Asian parids.
One example has been documented thus far of commonly occurring reversals of note ordering rules (where, for example, calls have both a note type 1–note type 2 order, and a note type 2–note type 1 order): In 1994, Jack Hailman of the University of Wisconsin documented this variation in the call of the black-lored tit, Parus xanthogenys, of India. The finding is an exciting and potentially important one: Vocal flexibility of this kind would greatly increase call complexity, and it has the potential to increase the variety of meaning receivers could obtain from calls. Such ability might also bring the call closer to the notion of syntax in human language—in which, for instance, “the child spoke to the toy” has a very different meaning than “the toy spoke to the child.” However, we can say very little about the potential pressures influencing the call system of the black-lored tit because so little is known about its social behavior or about closely related species in this geographical area.
We hope that this article will inspire increased efforts at understanding the social and vocal behavior of parids—such understanding is needed to determine the evolution of signaling complexity in these species. Furthermore, greater knowledge of the pressures shaping the chick-a-dee call system just might tell us a little more about the pressures that shape and constrain our own complex vocal system.