The Complex Call of the Carolina Chickadee
What can the chick-a-dee call teach us about communication and language?
The Structure of the Call
Chick-a-dee calls across parids share a number of acoustic features, each of which can be seen as somewhat analogous to aspects of human language. First, calls are composed of distinct note types. These note types have been categorized into acoustically distinct forms that can be distinguished by researchers with high reliability. In a 2012 study, two of us (Freeberg and Lucas) described six note types—A, E, B, C, Dh, and D notes—in the calls of Carolina chickadees from an eastern Tennessee population (see Figure 3). These note categories do not correspond to human musical notation; they are arbitrary labels. Christopher Sturdy and his colleagues at the University of Alberta have described a similar set of notes in the calls of Carolina chickadees and other chickadee species.
A, E and B notes are whistled and often show considerable frequency modulation. The C note is a noisy note type that generally increases in frequency over the course of the note. The D note, another noisy note type, has minimal frequency modulation. It seems to be a complex combination of two tones, or fundamental frequencies, and their harmonics, tones whose frequency is an integer multiple of the fundamental—along with other tones resulting from these tones’ interaction. (The songbird syrinx, or vocal organ, vibrates in two locations, one in each bronchus. Thus it can create two different tones simultaneously.) The final note type we described, the Dh or hybrid D note, is rare in this population and appears to be an A or B note that transitions without a break in sound into a concluding D note.
Each note type normally occupies a specific part of the call. The typical chick-a-dee call in this population has an average of two introductory notes (some combination of A, E or B notes), roughly one C note, and three concluding D notes. Thus, the chick-a-dee call is made up of note types with distinct sounds, similar to the way each human language is made up of phonemes, or distinct sounds. (For example, the p and b sounds in English are distinct phonemes produced by the lips, called labial stop consonants; the difference between the two is that the b is voiced, or articulated by vibration of the vocal cords, and the p is not.)
Second, chick-a-dee calls are produced according to rules of note ordering. Roughly 99 percent of a sample of over 5,000 chick-a-dee calls followed the A– E–B–C–Dh–D ordering rule. Any note type can be repeated or left out of the sequence. So the chick-a-dee call has constraints on how the different sounds that make it up are combined to form calls, a phenomenon perhaps analogous to human-language constraints that govern how different phonemes are combined to form words.
A third commonality among chick-a-dee calls is that the call system is open-ended. The more chick-a-dee calls we record, the more calls with different note-type compositions are revealed. This variation is possible because notes can be repeated in calls, within the constraints of the note ordering rules. We know this from analysis, based on information theory (the study of the quantification of information, begun in the 1940s), of calls recorded from the Tennessee population we have studied. The phenomenon is also supported by within-individual analysis of chick-a-dee call note types derived from large sets of calls of known individuals recorded over time. This open-ended quality is one of the major differences between the chick-a-dee call and the finite call and song repertoires of most songbird species. Open-endedness is one of the defining features of human languages.
A final common characteristic among chick-a-dee calls is that they contain a large amount of information. In information theory, this term refers to the amount of uncertainty in a signaling system. When a signaler produces a signal, the information in that signal reduces the overall uncertainty to the receiver about the context of the signal—in other words, the receiver knows more about the signaler or the signaler’s likely behavior than it did before the signal was produced. Signaling systems with a large amount of information therefore can conceivably transmit a wide variety of distinct messages. The greater information content in chick-a-dee calls stems from the enormous diversity in their note-type composition. A key assumption of the concept of information as it is typically used by parid researchers (and other bioacoustics researchers) is that diversity of note composition relates to distinct messages in signals. Evidence from different labs and from different chickadee species indicates that the variation in chick-a-dee call structure documented via information-based analyses does indeed correspond to functional variation. Certain note-composition variants in these calls seem to be messages, often to flockmates, about the social and physical environment or the behavioral tendencies of the signaler.