Duet-singing birds in South America's Andes mountains are helping scientists understand how the brain co-ordinates itself to co-operate with other individuals.

The plain-tailed wren, a small brown and grey bird that lives in bamboo thickets in cloud forests mainly in Ecuador, has impressed scientists ever since they realized that the bird's rapid-fire tweeting song is actually a duet, where the male and female alternate notes.

"It was absolutely amazing to us the first time we heard it," said Eric Fortune, a behavioural neuroscientist at Johns Hopkins University in Baltimore, Md., who wanted to uncover how the birds managed the feat.

Fortune and his colleagues flew to Ecuador and trekked more than two hours to the Antisana Volcano to find the plain-tailed wrens and record their songs.

"They are extremely loud singers," he told CBC's Quirks & Quarks in an interview set to air Saturday. "If you're near them, it's almost an unpleasant experience."

Eric Fortune sits in front of a system designed to record signals from the brains of plain-tailed wrens at the Yanayacu Biological Research Station and Center for Creative Studies in Ecuador as it was being built. Courtesy of Eric Fortune and Melissa Coleman

Fortune played a recording of the duet, which initially sounds like a single bird twittering. Then he played the songs of the female alone, a sparser series of notes that leaves gaps for her male partner to insert his part.

The scientists don't know why the birds co-operate on a single song. Like non-duetting birds, they seem to use singing in order to defend their territory. But Fortune said females may also be testing a male's ability to follow her lead.

"It's a test of their brain power, of their ability to produce this rapid-fire duet performance," he suggested.

To figure out how the birds' brains co-ordinated the duet, Fortune and his team captured some of the wrens and brought them back to the lab. There, they put the birds under anesthesia and placed an electrode in the part of the brain that controls singing.

When they played the songs back to the bird, they expected the bird to respond most strongly to its own song.

In fact, the birds' brains were most active when they heard the full duet.

"It means that they understand the output of their co-operative behaviour," Fortune said. "These birds have a memory of what they're trying to do together."

Fortune said the brains of all vertebrates are quite similar, and it is likely that they all wired in a similar way to carry out co-operative activities.

The research findings were published Thursday online in the journal Science.