23 JUNE 2005
Warning calls: Bird do it
Plenty of animals warn their groups about danger. Vervet monkeys even have different "words" for snake, eagle and leopard. Birds have warning calls. The black-capped chickadee, for example, is known to use "chick-a-dee" to arouse the flock to harass, or "mob," a predator like a hawk or an owl.
Photo: Chris Templeton, University of Washington
But can the chickadee give shades of warning? Can it notify its fellows that a looming predator is an especially dangerous species? Yes, according to research published today in Science. Instead of a simple "mob that bird," chickadees appear to utter something closer to "that's a really dangerous bird. Let's harass it until it leaves!"
The research was a lucky distraction from his original project, says Chris Templeton, who's now a graduate student at the University of Washington. He started the research project, as a masters student at the University of Montana, intent on seeing if chickadees could recognize the call of individuals in their flock.
Then he noticed that the alarm calls varied according to which type of predator was around. Scientists already knew that chickadees uttered a high-pitched "seet" when a predator was overhead, and used their characteristic "chick-a-dee" call to, among other things, alert flock-mates to mob a threatening bird that was perched. "We knew it was used as a mobbing call, to attract your friends, your flock-mates, to harass a predator, try to drive it out of your territory," says Templeton.
Chickadees do it
To investigate the response to predators, Templeton put flocks of six chickadees in an enclosure and recorded what the chickadees "said." In the presence of a harmless quail, or no extra bird at all, the chickadees gave no alarm. But when any of 15 species of raptors (including hawks and owls) entered the cage, the alarms began. The predator was tethered to prevent it from attacking the chickadees.
The alarms were more frequent when the especially dangerous saw-whet owls and pygmy owls were present. But the alarms also had a different sound, Templeton found: In the presence of small predators, the chickadees tacked an average of four "dees" to their call: "chick-a-dee-dee-dee-dee." When the larger, but less dangerous, great horned owl was present, they used two dees: "chick-a-dee-dee."
Chickadees warning about a low-risk predator, a great horned owl.
Chickadees warning about a dangerous pygmy owl.
mobbing a pygmy-owl in the wild. "Chick-a-dee-dee-dee-dee" and the high-pitched "seet" sound
are warning sounds with distinct messages.
Smaller predators are more dangerous because their greater agility helps them catch in-flight meals. "We think size is a pretty good proxy for threat. If you going to be able to eat a bird, you have to catch it, and that means you have to be highly maneuverable, and that means, according to the laws of physics, that you have to be small."
Using electronic analysis, Templeton heard other signals that would normally elude us. "They are changing a whole bunch of very subtle features that we can't even begin to hear. Our ears don't work fast enough or well enough to process that information," he explained.
Data from "Allometry of Alarm Calls..."
Although the chickadees are clearly responding to size, that would not explain why they ignored the quail, so it's likely they were responding to other clues, such as the particular species it encounters.
The ongoing struggle with predators gives chickadees an evolutionary reason to develop protective measures such as warning calls. Ideally, a warning would convey the severity of the hazard, and that appears to be happening, according to the analysis of 5,000 recordings of captive chickadees.
Dee is for danger
To prove that the "language" was conveying information among the chickadees, Templeton, working with Erick Greene, an associate professor of biology at the University of Montana, and Kate Davis, of Raptors of the Rockies, aired the recordings to the chickadees, and measured their degree of alarm. It worked: Recordings made in response to more dangerous raptors elicited more mobbing behavior, confirming that the chickadees understood the meaning of the calls.
Photo copyright Ross Warner
"'Chick-a-dee' turns out to be quite a sophisticated call," says Templeton. "It's used for a whole bunch of other purposes; to advertise that someone has discovered food, it contains information about individual identity, about the flock identity. There is tons of information, and that makes it even more surprising that they also are including subtle information about predators" in the same basic call.
While this may be the most sophisticated bird "vocabulary" found to date, Templeton suspects others are out there. "This is the most detailed communication we have found, but it is also the finest scale that anyone has looked. It would not surprise me if other species are able to do this. All these signaling systems are a lot more complicated than we really expect, until we spend a lot of time and energy looking at them."
And how long did Templeton spend checking out how chickadees warn the gang of danger? Three years.
-- David Tenenbaum
Allometry of Alarm Calls: Black-Capped Chickadees Encode Information About Predator Size Christopher N. Templeton, Erick Greene, Kate Davis, Science, 24 June, 2005.