The Complex Call of the Carolina Chickadee
What can the chick-a-dee call teach us about communication and language?
Changing Notes, Changing Messages
Individual parids are often out of sight of flockmates as they move through the environment, so a vocal signaling system that can convey messages related to predators, food or group movement seems crucial to obtaining the benefits of group living. Recent studies indicate that variation in Carolina chickadee chick-a-dee calls is associated with these social and environmental contexts (see Figure 4). Chickadees and other parids have a number of distinct call types in their vocal repertoires, but our focus here is on chick-a-dee calls, so we use “calls” hereafter to refer to chick-a-dee calls.
Most studies of these calls in the context of avian predators have used perched predators or models, as we along with Tatjana Krama and Cecilia Kullberg noted in a recently published review article. Christopher Zachau and Freeberg, in an article published this year, presented predator and control stimuli that “flew” in the area of Carolina chickadees visiting feeders. (See the sidebar on page 403 for more detail about the design of these experiments.) We used wooden models shaped like flying birds and painted to resemble either sharp-shinned hawks (Accipiter striatus, a threatening avian predator) or blue jays (Cyanocitta cristata, a nonthreatening avian control). The chickadees’ calls were recorded before and after the release and “flight” of the models down a zipline near the feeders. The calls produced varied with the presence of each model type, but the biggest effect we measured resulted from the flight of any model, irrespective of the species it mimicked. Calls produced after the model was released contained more A notes compared to calls produced prior to the release of the model. Greater production of A notes in the calls would seem to represent a message of alarm, as opposed to one of mobbing—behavior that is frequently linked to approaching and harassing predators—or of assembly. Tonal sounds that slowly increase in intensity and that are high frequency (such as the A note) are generally difficult for avian predators, and many other animals, to locate. In contrast, noisy sounds with rapid increases in intensity, like the D note, are easier to locate. Thus, the production of more A notes in these calls when a flying predator is detected in the area seems adaptive, as it could alert flockmates to the predator’s presence but not give away the location of the signaler to the predator.
Carolina chickadees produce more calls, and often more D notes in those calls, when they detect a perched avian-predator model than when no model is present. For example, in a 2009 study, Chad Soard and Gary Ritchison of Eastern Kentucky University placed six perched avian-predator models in the habitat of Carolina chickadees. The models, all of which represented hawk and owl species, ranged in size and type from small, agile predators like Eastern screech owls (Megascops asio) and sharp-shinned hawks to large, relatively slow-moving predators like great horned owls (Bubo virginianus) and red-tailed hawks (Buteo jamaicensis). The former predators represent real threats to small songbird species, whereas the latter do not. Chickadees produced more D notes in their calls when smaller, more threatening avian predators were present (see Figure 5). Later the researchers played back chick-a-dee calls recorded in these different threat contexts to chickadees in their habitat. The authors found that chickadees were more likely to mob the playback speaker—to approach it closely in large numbers—when it was playing calls recorded when a small predator model was present than when the speaker was playing calls recorded when a large predator model was present. This work suggests that easy-to-localize D notes are used more often in calls when those calls might serve a mobbing function—bringing flockmates to a particular location to drive a predator away. These findings make it clear that Carolina chickadees vary the note composition of their chick-a-dee calls in the high arousal contexts of predator detection and mobbing.
Ellen Mahurin and Freeberg found in a 2009 study that when individual chickadees from an eastern Tennessee population first detected food, the calls they produced contained a relatively large number of D notes (see Figure 6). Once at least one more chickadee arrived at a feeder, however, the first bird’s calls changed such that fewer D notes were produced. In a follow-up study near feeders at several sites, we played back calls that contained either a large number of D notes (which previous findings suggested might be a signal to assemble) or a small number of D notes (as a control). Carolina chickadees flew to and took seed from the feeders more quickly in response to calls containing a large number of D notes, supporting the notion that increased production of D notes can help recruit other individuals to the signaler’s location.
A naturalistic observation study conducted by Freeberg in 2008 suggests that chickadees use more C notes in their calls when they are in flight than when they are perched (see Figure 7). We have recently gained more experimental support for this suggestion: Chickadees flying to and from feeders produce calls with a greater number of C notes than they do when they are farther away from feeders. In addition, chickadees released from capture produce calls with a greater number of C notes when they are in flight than they do once they are perched. So calls with a relatively large number of C notes might signal movement—and thus might be adaptive for maintaining group cohesion in space.
In addition to these environmental and behavioral contexts, we have detected motivational influences on call production: Lucas, April Schraeder and Curt Jackson found in a 1999 study that chickadees increase rates of chick-a-dee calls when their energy stores decline. Additionally, there appear to be population-level “signatures” in the call that distinguish one population from another. There also appears to be marked variation at the individual level in call production. Evidence from Christopher Sturdy’s lab at the University of Alberta indicates that individual Carolina chickadees, as well as a number of other chickadee species, can often be statistically discriminated from one another by virtue of the acoustic characteristics of the note types of their calls.
We thus have considerable evidence that the note composition of calls of Carolina chickadees is associated with detection of predators (both perched and flying), food detection, individual flight and motivation. The calls also vary in ways that may suggest markers for individual, flock, population or some combination of the three. Variation in the note types that make up the call corresponds to different contexts and to population-level characteristics. Studies of call variation have also been carried out in other parid species. For example, as a 2012 review article by Krams and coauthors reveals, perched predator contexts have been shown to have a similar effect on call note composition in black-capped chickadees, Mexican chickadees (Poecile sclateri) and willow tits. Call variation seems to be associated with food contexts in black-capped chickadees and with flight contexts in mountain chickadees (P. gambeli). Krama, Krams and Kristine Igaune in 2008 documented variation in the comparable call system in crested tits (Lophophanes cristatus), based on whether individuals were close to the relative safety of vegetation or were exposed in open areas away from cover. Another interesting finding about this species is that dominant individuals use their calls differently than subordinate individuals, which suggests possible personality-like influences on call variation.