FOCUS ON FIRE
IN THE FOREST
FUNCTION OF FIRE
A GRASP ON SMOKE
BEYOND THE BLAZES
UM research reveals s0phisticated bird calls
By Cary Shimek
The black-capped chickadees had lived an exciting life of late.
First, researchers netted them, and the little birds suddenly found themselves living in a large aviary attached to UM’s Field Station at Fort Missoula. Wire mesh kept them from flying away, but the aviary contained bushes, trees and plenty to eat. So the chickadees foraged, slept and sang.
Then things turned surreal. You see, a Field Station window opens into the aviary, and a little stage had been built on a stump before the window. And scientists had set up a curtain around the stage.
One day the curtain opened to reveal … a pygmy owl!
Now, nothing ruffles chickadee feathers like a pygmy owl — the Great Satan of their world. The little birds can fly circles around a ponderous bald eagle or great horned owl, but a pygmy owl is unbelievably quick and lethal.
So the flock loosed their trademark call: “chick-a-dee-dee-dee-dee, chick-a-dee-dee-dee-dee.” It’s a mobbing call — lots of birds have them — and it means something like, “Hey, this is really dangerous. You have to help me out here and pay attention to this.” It also means, “Let’s get it!”
With this call to action, the chickadees dive-bombed the little owl en masse. They made lots of noise, swooped at it and even tried hitting the owl in the back of the head. They wanted to harass the raptor and get it out of there.
Researchers soon took the owl away, but the curtain opened again and again in coming days and weeks to reveal more surprises: a great gray owl, a peregrine falcon, a merlin hawk, a ferret and others. With each unveiling the chickadees assessed the threat, uttered a call describing the situation — often with nuances beyond human hearing — and then reacted appropriately. They didn’t know their specialized calls were recorded.
At times the flock would hear the distinctive chickadee calls for a pygmy owl or kestrel hawk, but when the birds mobbed the area where the call came from, all they found was an audio speaker.
Yes, life for the chickadees had definitely become more interesting.
Erick Greene is a UM professor, behavioral ecologist and leading expert on the language of birds. In recent years his science team studied local chickadees to discover a previously unknown level of sophistication in their calls. His partners were Chris Templeton, a former UM graduate student now working on a doctorate at the University of Washington, and Kate Davis, director of Raptors of the Rockies.
They found that chickadees, which are common across North America, have a complex system of alarm calls that convey information about the size and danger of potential predators and tell the birds what sort of defense to mount in response.
Their work was notable enough to be featured in a summer issue of Science, one of the world’s top research journals, and from there the story hit the New York Times, the Washington Post and media outlets worldwide.
Greene says their hundreds of hours of experiments over several years would have been impossible without the assistance of Raptors of the Rockies, a Florence-based nonprofit organization that cares for disabled birds and uses them for wildlife education programs. Davis, the organization’s director, co-authored the Science article. She cares for birds that haven’t been injured badly enough to be put down, but enough that they could never survive in the wild.
In fact, the chickadees might be mortified to learn the “Great Satan” pygmy owl they found so terrifying is a wounded bird named Dot Com.
Greene says little birds usually have two types of alarm calls. Their “seet” call — a soft, high-pitched noise that sounds like “sssst” — alerts their fellows to predators flying overhead. It tells the birds to beware and take cover.
Many small birds also use a mobbing call, which is given around stationary or perched predators that are perceived to be less of a threat. The cry means it’s time to gang up on the threat and drive it away.
Greene says humans have studied the mobbing reaction of birds for thousands of years. He is quick to describe a Greek vase from 600 B.C. that depicts a tethered owl being mobbed by birds while a human bird-catcher looks on. He says that sometimes birds of several different species will be summoned to a single mobbing display.
“With our research, we wanted to see if there were any differences in this historically well-documented phenomenon of mobbing,” Greene says. “We wanted to test the idea that mobbing calls to other birds reflect different levels of threat. “And, indeed, it does. This was completely unexpected.”
Greene and his fellow researchers exposed the chickadees to 13 different raptor species in a variety of sizes, as well as a ferret, weasel and cat. They also showed a seed-eating bobwhite.
“The bobwhite was the control,” he says. “It’s harmless; not a raptor at all. We wanted to make sure that the chickadees weren’t giving alarm calls for just anything.”
From thousands of recordings analyzed with the help of a computer, the researchers found significant variation in the “chick-a-dee” mobbing calls. Essentially, if something is more dangerous to them, the birds make more mobbing calls.
They also tack on more “dee” syllables to the end of the calls when confronted by a predator perceived to be more dangerous.
UM scientists found smaller predators generally are more alarming to chickadees than larger ones. A massive great horned owl, for instance, usually can’t turn quickly enough to chase down highly maneuverable chickadees, so it only elicited two “dee” syllables at the end of calls. But the sight of a lightning-quick pygmy owl elicited four “dees.”
Besides small owls and hawks, the chickadees became frenzied over ferrets and cats.
Delving beyond the extra “dee” noises — which people can hear — the team used specialized analysis software to discover hidden acoustical differences. Sifting through the frequencies and harmonics of the calls, the UM researchers discovered a lot of other differences that vary systematically.
“We found that chickadees are encoding much more in these acoustic signals than we can appreciate with our hearing,” Greene says.
The team repeated their experiments many times with different flocks of chickadees to ensure the birds were using the same calls for similar threat levels. They also randomized the order in which they showed raptors and other predators to the birds.
“Also, in the other half of our experiments, we tested whether chickadees were actually getting information from the calls,” Greene says. So the team did playback experiments using speakers hidden in the aviary. They played the recordings when there were no predators present.
“It got their attention immediately,” he says. “And we showed that the intensity of the mobbing response varied depending on what recording we played. If we played one recorded in response to a pygmy owl, it got a much more intense mobbing response. Boom, they would come and approach the speaker. They would move closer and spend much more time investigating the area. They would stay longer in the area.”
Greene has worked at UM for 15 years, and his work on animal communication has been published in Science or Nature five times. He says the chickadee experiments demonstrate that bird communication systems are far more sophisticated and flexible than previously suspected.
He’s no Dr. Doolittle, but Greene admits to becoming somewhat fluent in chickadee.
“Now, when I hear chickadees upset, my ears are tuned in a little more,” he says. “I can say, ‘Yeah, there is something over there, but they aren’t too upset.’ Or I can hear, ‘Whoa, something has them really riled up.’”
For more information, e-mail Greene at email@example.com.