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February 7, 2006

Bird Mimics Other Species in the Sri Lankan Rainforest

This is part of a series on recent Sigma Xi grant recipients. The next Grants-in-Aid of Research application deadline is March 15. Visit link for details.

Click here to listen to sounds of the rainforest.

RESEARCH TRIANGLE PARK, NC - While watching mixed-species bird flocks in the Sri Lankan rainforest over the last decade, Eben Goodale and Sarath Kotagama were sometimes mystified to hear what they thought was a flock, only to find a greater racket-tailed drongo perched all by itself.

Drongo bird It took them a few years to realize that drongos could make all the noise themselves by imitating other species.

"I still remember vividly the moment that I first observed this behavior," Goodale says. "I was following a mixed-species flock through the rainforest. These flocks are large and noisy, averaging 12 species and nearly 40 individuals."

From the back of the flock, one bird, a greater racket-tailed drongo, swooped down and approached him to within three meters at head height. "The drongo was clearly mobbing me--a behavior that birds use to notify other individuals of the presence of a stationary predator."

The familiar scene quickly changed, according to Goodale. "The drongo did an extraordinary thing: it began to mimic the mobbing-specific note types of other species. It kept rotating through the mobbing notes of other species, in addition to its own notes."

Goodale wouldn't have understood what was happening if he hadnít just completed a study on the alarm-associated calls of all the species in the flock system.

An animal behaviorist who just finished his Ph.D. at the University of Massachusetts, Amherst, Goodale says the two Sigma Xi Grants-in-Aid of Research he received have been invaluable to his work, contributing much to the success of the project.

Eben Goodale recording birds. It was the way drongos mimic that the researchers found surprising. In an article last year in the British journal Proceedings of the Royal Society: Biological Sciences, they reported that when a drongo is alarmed, it doesn't merely make the alarm calls of its own species. It mimics the alarm calls of other species as well and appears to know just the right call for a given situation.

This marks the first time, the two researchers say, that a cognitive ability of this kind has been seen in birds in the wild.

"While many birds copy the songs of other species," Goodale says, "there usually is not a connection between the sound they mimic and the situation that they are in. They may mimic alarm calls, but not use them in the appropriate situation."

He and Kotagama, a professor at the University of Colombo, Sri Lanka, have worked together since he was an undergraduate at Harvard and then a Fulbright Junior Scholar.

In studying drongos, the two first developed a "dictionary" of drongo calls made when drongos were attacked by predators.

Since drongos combine mimicry with their own calls, it's possible to see that when drongos are alarmed, they mimic the alarm calls of other species as well as the calls of nest predators, eagles and hawks.

"Although we are not yet sure why drongos imitate in this way, we suspect that by using other species calls or the calls of dangerous predators, they can communicate about a threat directly with other species," Goodale says. "That's the hypothesis we're now hoping to test."

He continues, "What's fascinating about this behavior is that it's reminiscent of what we know some birds--notably parrots--are able to do in the laboratory: learn to use other species' signal in a context-dependent manner. But such context-dependent mimicry has not yet been demonstrated in the wild."

"Itís very significant," says fellow Sigma Xi member Bruce E. Byers, a UMass biology professor and one of Goodale's thesis advisors. ďThe idea that signals could have evolved beyond species boundaries hasnít really been demonstrated before."

Byers says Goodale's paper also reveals "the pretty extraordinary cognitive abilities of the drongo," an area that his former student is continuing to investigate.

Other aspects of Goodale's research were published last year in Auk and the Journal of Tropical Ecology.

Goodale and Kotagama have conducted their field studies primarily in the Sinharaja World Heritage Reserve, the largest tract of lowland rainforest remaining in Sri Lanka, home to many of the country's endemic and endangered plants and animals.

The Sigma Xi Grant-in-Aid recipient is planning further fieldwork in Sri Lanka, India and Papua New Guinea.

So far, this line of research has led to the production of a CD called Sounds of Sinharaja: A Day's Walk Through a Rainforest in Sri Lanka. Proceeds from its sale go to the student fund of the Field Ornithology Group of Sri Lanka.

Click here to listen to sounds of the rainforest.


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