Slater and colleagues Nigel Mann at State University of New York College at Oneonta and Kimberly Dingess at Indiana University in Bloomington first discovered the complexities of the wrens' song last year.
The team, led by Mann, describes its findings in the March 2006 issue of the journal Biology Letters.
The group made the discovery by chance while trapping birds as part of a larger project to determine the relationships among some 28 species of wren that live in Mexico, Panama, Costa Rica, and Ecuador.
The researchers played recordings of birdsong to lure the wrens into nearly invisible net traps. The wrens are territorial and so rush into the nets to scare off the supposed intruder.
"Usually within one or two minutes, we've got the territory owners arriving, and that's exactly what happened," Mann said today in a broadcast of the Pulse of the Planet radio program.
(Pulse of the Planet and this related news series receive funding from the National Science Foundation.)
The researchers watched from the bushes as one bird arrived at the net, followed moments later by a second.
Normally wrens live in pairs. With two plain-tailed wrens captured, the researchers thought their work was done.
"But before we even had a chance to get up and extract these two birds out of the net, a third flew into the net. And then a fourth," Mann said.
At first the team thought they might be on a boundary between two territories, and both sets of owners had converged on the trap.
"Then a fifth bird flew into the net and then a sixth and then, finally, even a seventh bird. And this had us totally baffled. We never had this with any of our other wren species before," Mann continued.
Sharon Gill is a postdoctoral fellow in ecology and evolutionary biology at Princeton University in New Jersey who studies the behavioral ecology of temperate and tropical birds, including wrens.
She says that it's hard to tell if more than one bird is singing during a duet, except that the sound is fuller.
"What strikes me in this paper is what it must have felt like to discover up to seven birds singing [at once] That must have been a spectacular experience to witness," she said.
The next step for the bird researchers, Gill adds, is to understand why tropical birds sing duets, and why some species such as the plain-tailed wrens live and sing in larger groups.
According to study co-author Slater, the chorus singing is likely a territorial defense meant to intimidate any would-be intruder.
"I liken it to what the New Zealand rugby team calls a haka," he said.
In the haka, a Maori tradition, the rugby players get in formation, jump up and down, and chant. The ritual is meant to prepare the players for the game and intimidate their opponents.
The chorus singing could also help synchronize breeding, Slater says.
Most birds use seasonal daylight cues to stay in tune with each other. But the plain-tailed wrens live very close to the Equator, where day length is the same year-round.
By singing together, the birds could be keeping each other in tune through the various stages of their breeding cycle.
Princeton's Gill says the singing probably combines all these functions.
"I think we need to do more work to fully understand it," she said.
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