Biologists report new progress in translating the sophisticated communication system of black-capped chickadees.
When the little birds spot a lurking predator, they burst out with variations on their “chickadee” calls. Tests with 15 predator species show that birds vary those calls depending on how dangerous the predator is, says Christopher N. Templeton of the University of Washington in Seattle. For example, the greater the danger, the more dee syllables the birds tend to add at the end of “chicka.” Recordings of these varied alarm calls inspired appropriately mild or intense responses in a chickadee flock, the researchers report in the June 24 Science.
Alarm calls have provided a valuable model system for scientists decoding animal communication. The only previous experimental study of specific information in bird alarm calls found that chickens cluck warnings for danger from above that differ from warnings for danger on the ground.
Templeton says that in the recent test, he at first showed a stuffed hawk to chickadees in an outdoor aviary, but they were only once fooled into voicing alarm calls. Subsequently, he had to borrow live raptors from a bird-rehabilitation center.
Chickadees that spot an approaching predator rapidly give high-pitched “seet” calls and flee. For stationary predators, chickadees voice variations on “chickadee” and mob together to harass the menace.
Templeton staged the appearance of stationary predators, including various raptors, a cat, and a ferret. Among the raptors, the greatest dangers to chickadees come from small species such as the coffee cup–size pygmy owls.
Great horned owls have fearsome beaks and talons but are “big and clunky,” says Templeton. A little chickadee can usually dart away.
After analyzing more than 5,000 alarm calls from the chickadees, Templeton and his colleagues report that the calls differ in response according to the predator. For instance, calls triggered by small, dangerous raptors tend to have shorter intervals between “chick” and “dee” and extra dee syllables, on average four instead of two. However, in one case, a really scary pygmy owl rated 23 dees.
The cat, ferret, and kestrel triggered calls having intermediate characteristics.
To see whether such distinctions matter, the researchers played back alarm calls to chickadees when no predator was in sight. Calls recorded from a pygmy-owl sighting seemed to upset the flock more, prompting more birds to come close to the speaker, than did calls recorded during the relatively ho-hum appearance of a great horned owl.
Dan Blumstein of the University of California, Los Angeles, who has studied alarm calls in marmots, praises the careful design of the chickadee experiments, which he calls a compelling demonstration of complex communication among birds.
Comparative psychologist Todd Freeberg of the University of Tennessee at Knoxville, who has studied chickadee communication, says that Templeton’s work is “really starting to chip away at understanding the richness of the information” that chickadees can communicate.
