WASHINGTON — Those cheery-sounding chirps coming from the tree in the back yard are carrying more than a joyful message -- they are conveying surprisingly complex information about lurking predators, biologists reported Thursday.

Tiny chickadees, known for their scolding calls, communicate details about nearby predators, biology PhD student Chris Templeton of the University of Washington found.

For instance, the final, or "D" note in a call can be repeated for emphasis, Templeton said.

"If you ever go out and are hearing a chickadee making a really long string of "D" notes on a call -- six or eight or even 10 -- you know there is a really dangerous predator around, maybe the next-door neighbor's cat or an owl or a fox," Templeton said in a telephone interview.

"It is very strongly correlated with predator body size."

And with chickadees, smaller equals more dangerous, Templeton wrote in a report published in Friday's issue of the journal Science.

"The only predators that can really catch chickadees are the ones closest to them in size," Templeton said.

Captive chickadees reacted strongly when shown a pygmy owl, for instance, but their calls communicated less alarm when they saw a larger raptor.

The social birds often mob a predator to drive it away.

"A great horned owl going after a chickadee would be like a Hummer trying to outmaneuver and catch a Porsche," Templeton said.

The tiny black-capped chickadee, with ranges across much of the United States and Canada, has been studied for more than a century, so why hasn't anyone noticed before now?

"In the chickadee call there are a lot of very subtle variations, most of which we actually can't hear," Templeton said. "They change a lot of features in the call that you can see in a sonogram."

There are overtones in the "D" notes of the characteristic "chicka-dee-dee" call that helped name the little songbirds, relatives of tits.

The calls, rapid-fire to a human ear, also contain subtle spacing and timing of notes that the birds can vary, Templeton said.

"The one thing that we can hear is the number of "dee" notes at the end of a call," he said.

"It looks like the more dangerous a predator the chickadee encounters, the more dee notes it has."

When recordings were played back to chickadees, they showed "mobbing" behavior appropriate to the predator that the recorded bird had been seeing, Templeton said.

The research joins a growing body of studies that suggest birds use complex communications and tools. Studies have shown that birds dream, rehearse their songs and have regional dialects.

"I guess the take-home message is this is probably true of many animals. Many animals communicate much more sophisticated information than we realize because we are not in a position to understand their language," Templeton said.

Source: Reuters