Mobbing Calls of Black-Capped Chickadees

"A tit can tell you not only that it is in danger, but what kind of danger it is in. It can say, 'Look out for the cat,' or 'Look out for the hawk,' or 'Look out for the tawny owl,' as plainly as A.B.C."

--Archimedes the owl. The Once and Future King, T.H.White

The black-capped chickadee (Poecile atricapilus) is a one of the most ubiquous and best loved birds in North America. This species has also been one of the most widely-studied of birds. One reason they have been so well studied is because they have a very complex social system that includes both summer territoriality and winter flocking. A reason that they might flock in the winter is to decrease their chances of being attacked or eaten by a predator. In the winter months in Montana, chickadees are faced with a wide variety of different predators that vary tremendously in the amount of risk they pose. One way they communicate information about predators is with the "chick-a-dee" vocalization.

The "Chick-a-Dee" Call

The "chick-a-dee" call is composed of four types of syllables (A, B, C, D), which typically occur in this order. Listen to a recording of the pictured call. To be able to distinguish among the note types, it is necessary to listen to the call slowed down. The number of syllabes and the acoustic structure of these syllables is highly variable among different "chick-a-dee" calls. Some of this variation apparently communicates information about individual and flock identity. Because this call is also used to communicate about predators, some of the acoustic variation may also serve to code information about the type or level of threat of the predator being mobbed.

Predator Presentations

To determine whether variation in the "chick-a-dee" call is related to the type of predator being mobbed, Erick Greene and I conducted predator presentations to flocks of chickadees living in a large, outdoor, semi-natural aviary. Predators included 15 different species of live mammals and raptors. Working with Kate Davis and the Raptors of the Rockies gave us access to a great number of different species of live raptors that ranged 10-fold in body size from the tiny Northern Pygmy-owl up to large owls and hawks. For each presentation, we recorded the behavior and the "chick-a-dee" mobbing vocalizations of the chickadees. Each vocalization was then analyzed to determine the number of each type of syllable and other, more subtle, acoustic features like the length of syllables and the number and spacing of overtones in the D notes.

photo by Kate Davis

Many features of the "chick-a-dee" call were highly variable among the calls I recorded, including the number of syllables, the spacing of the notes, and fine-scale acoustical features of the "dee" syllables. These variations correlate strongly with the degree of threat posed by a given predator. For example, a low risk predator, like a great horned-owl or a rough-legged hawk, elicit calls that have fewer, longer D syllables, that are more widely spaced and have more overtones than those calls produced in response to a more dangerous predator like a pygmy-owl or merlin.


Low Threat Alarm Call	High Threat Alarm Call

photo by Erick Greene

The Allometry of Alarm Calls

Potential predators vary tremendously in body size--for example the Great Horned-owl is more than 10 times heavier than the Northern Pygmy-owl. Chickadees may use this difference in size to assess the risk of predation. Spectrographic analyses indicate that the "dee" syllables of the "chick-a-dee" mobbing call are directly related to the size of the predator encountered. Small predators elicit calls with many D notes, while large predators elicit calls with few D notes. Interestingly, the natural history of each predator seems also to be important. For example, large predators that eat many small birds are above the line while small predators who eat few birds are below the line. This suggests that chickadees actually encode information about predator threat in their alarm calls.

Owls as appears in Science (308): 1934-1937

Playback Experiments

To test whether chickadees actually use the subtle information about predators encoded in their mobbing calls, I also conducted playback experiments. I presented vocalizations that were recorded in response to both nothern pygmy-owls (small, high threat) and great horned owls (large, low threat). Chickadees responded much stronger to the high threat mobbing calls than the low threat calls, by approaching the speaker and increasing their vocalization rates. These results indicate that the information encoded in the "chick-a-dee" call is actually used for communicating information about predator risk to other flockmates. Together, these results highlight one of the most sophisticated and subtle communication systems yet discovered.

Templeton, C.N., Greene, E. & Davis, K. 2005. Allometry of Alarm Calls: Black-capped Chickadees Encode Information About Predator Size. Science (308): 1934-1937.

Department of Biology