By Harrison Jones, Masters student, Wildlife Ecology & Conservation Dept.
Ask anyone with a birdfeeder and they might agree that the chickadee is the preteen of the songbird world. Moving around in cliquish groups, infinitely excitable, and talkative to a fault, these birds would fit in well at your local Middle School. Yet what might seem like the inane jabbering of a small bird to some has become the subject of over a decade of intensive study by Dr. Todd Freeberg, a scientist at the University of Tennessee. In fact, Dr. Freeberg has helped uncover what may be among the most complex of animal languages in the Carolina Chickadee (Poecile carolinensis), a common backyard bird throughout much of the eastern United States. In the fall, these little birds transition from breeding pairs to winter social groups of 2-6 chickadees that forage and move together throughout the forest. Perhaps because of this social nature, chickadees have evolved a huge diversity of calls to communicate with their flockmates, the meaning of which we are only just beginning to understand.
Dr. Freeberg has focused his research on the chick-a-dee call – one thought to be used for social communication and from which these birds get their names. The similarities between this bird call and human language are immediately obvious. Much like a sentence, the overall call is composed of a series of different note types that form the words of “chickadese”. The birds are able to experimentally differentiate between the seven known note types, and can add even more complexity by repeating the same note several times in the same call. Another parallel with human language is the use of ordering rules, or the grammar of the chickadee language, which appear to limit where in the call certain note types can be placed. The A and C notes, for example, are placed at the beginning of the call, while other D notes are found at the end. And just as a human might avoid ending a sentence with a preposition, a chickadee would never, say, place an A note after a D note. In fact, chickadees appear to be much better at grammar than middle schoolers: Dr. Freeberg has found that less than 0.4% of calls violate these ordering rules.
But perhaps the most interesting finding about chickadee language has been that their calls are an open-ended system. In most songbirds, the number of calls produced are finite – given enough time, one could find and record all of the call types produced by a given species. But because the chick-a-dee call notes can be combined and repeated in myriad ways to produce different meanings, an attempt to record all of the chickadee’s calls would constantly yield new combinations of note types, thus making it impossible to record all of their calls. The same is true of humans of course. We can combine words in endless ways to make sentences, generating an infinite number of possible sentences.
To add even more complexity to the equation, chickadees appear to have regional dialects. Dr. Freeberg recorded flocks in both Tennessee and Indiana, and found that important regional differences exist in the frequency of different note types used in calls: for instance, a common note type in Tennessee (D) was nearly absent in the Indiana population. So not only is extensive variation occurring within populations, but even between regions of the country!
What might chickadees need to say to each other that would require the evolution of such a complex animal language? Dr. Freeberg has been spending the past decade trying to find out. He has focused his efforts on understanding what the individual note types used in the chick-a-dee call might mean, which often requires understanding when they are being used. For example, the D note is often produced when chickadees see a predator. Studies have shown that they repeat many of these D notes at the end of the chick-a-dee call when they encounter a perched predator. When researchers presented captive chickadees with birds of prey of different sizes, they found that the number of D notes given in the call were correlated with the body size of the predator. Small predators such as screech owls elicited large numbers of D notes while larger hawks and owls saw relatively fewer D notes produced. Because only small birds of prey are likely to attack a chickadee, the number of D notes effectively conveys how dangerous a potential predator might be based on its body size.
The D note also appears to be used when chickadees come across new food resources, such as a new birdfeeder. Chickadees add more D notes to the end of their calls when they are the first to find food compared to when another bird has found it before them. When Dr. Freeberg and colleagues played recordings of calls with more D notes in them, they attracted wild birds much more quickly than calls with fewer D notes. Why are the number of D notes related to both food and predators? Dr. Freeberg believes it may have a lot to do with recruiting other chickadees to a particular site. When chickadees find a dangerous predator they often band together to harass and drive it away, in a behavior known as mobbing. When a chickadee finds a new food resource, it might want to attract other birds in its flock to the food source in order to share it with them. One of the advantages of flock living may be that birds can share knowledge about food sources with each other, helping them survive often harsh winter weather. So in both of these high D note contexts it is beneficial for chickadees to attract other group members.
If the D note may be a signal for recruitment, then the A note, often given at the beginning of the chick-a-dee call, may be its polar opposite. Dr. Freeberg has found that when dangerous hawks fly through or near a flock, and birds must escape the predator, the number of A notes produced in calls is increased. In order to test if the A note is really associated with alarm and aerial danger, he conducted an experiment with wooden bird models that were ‘flown’ over foraging chickadees. When presented with a dangerous sharp-shinned hawk model, wild chickadees were more likely to produce more A notes than when a model of an innocuous blue jay was flown overhead, supporting the theory that this note is associated with alarm. This is reflected in the structure of the note itself: the A note is a relatively pure tone with a high frequency, making it hard for an attacking predator to pinpoint exactly where the sound is coming from and keeping the calling individual relatively safe.
Finally, Dr. Freeberg has looked at the importance of the C note, another call note that might be associated with flight. But in this case, it is produced when the chickadees themselves are in flight. More C notes are thought to be produced in the chick-a-dee call when birds are in flight versus perched. In an experiment in which captured chickadees were released and their calls recorded, the birds gave more C notes in the immediate aftermath while in flight then later when they had landed on a perch. The result was the same when researchers walked towards feeding birds and recorded calls before and after birds took off, and when birds were recorded flying to and then perched at a bird feeder, suggesting that the C note indicates that an individual is moving.
Thus, the different note types of the chick-a-dee call each have different meanings, and can be combined to encode information about movement, recruitment, and predator attack simultaneously.
Our knowledge of animal languages is still in its infancy, but Dr. Freeberg’s research shows how animal languages can teach us a lot about our own. The study of animal languages shares many basic questions with those of linguists – how did language evolve and what determines how complex our language needs to be? The next step for Dr. Freeberg will be to understand what drives the evolution of this language – is it the dynamics of the social group, the kind of habitat in which the birds live, or the number or kinds of predators that they face? Only time will tell, but it is certain that there is much more to the language of these little birds then first meets the eye – a fact that doubtless also holds true for the middle schoolers in our lives.
Photo credit: bobloblaw/Thinkstock