From the Science Magazine Website:

"Dee" is for Danger

Great horned owls are fearsome predators, but for agile birds like chickadees, the smaller, more maneuverable pygmy owl is probably a bigger threat. Now, researchers report that black-capped chickadees have a sophisticated system of alarm calls that convey information about the size of potential predators. The calls appear to help the chickadees mount a coordinated defense that's calibrated to the danger.

Chickadees are social birds, and they gang up to "mob" predators and drive them away. "They dive-bomb the predator and make a lot of noise," says study author Christopher Templeton, currently a Ph.D. student at the University of Washington, Seattle. Templeton and colleagues suspected that the chickadees' calls might organize their defense. The birds make two kinds of alarm calls: a soft, high-pitched "seet" and the louder namesake "chick-a-dee." A previous study found that the "seet" call indicates the presence of predators flying overhead. The "chick-a-dee" call is used in a variety of situations, such as signaling the presence of stationary predators or identifying flock mates.

The researchers recorded more than 5000 calls that birds made when various predators--including owls, hawks, and falcons--were placed in their woodsy outdoor enclosure. The recordings showed that, among other changes, the birds made more "chick-a-dee" calls and incorporated more "dee" syllables into each call when they saw a small raptor with a short wingspan. A 70 gram pygmy owl, for instance, might elicit four "dees" at the end of a call, whereas a 1.4 kilogram great horned owl might elicit only two. But not all small birds elicited more "dees": a harmless quail provoked fewer than two "dees" per call, on average. The researchers hypothesize that more "dees" means more danger.

Next, the team played back recorded alarm calls through a hidden speaker and watched the chickadees' reaction. More birds mobbed the speaker for a longer period of time in response to a small predator alarm call than in response to a large predator call, the team reports in the 24 June Science.

The study provides a rare example of alarm calls that convey information both about the type of predator and the degree of threat, says Dan Blumstein of the University of California, Los Angeles. James Hare of the University of Manitoba in Winnipeg adds that "even very common species that we may take for granted have evolved very elaborate and exacting systems of communication."