Slater explained to Animal Planet News that up to seven birds sing "choruses," with males and females contributing different parts. The song consists of a series of four repeated phrases that follow the pattern ABCDABCD. Males sing A and C, while females sing B and D.
All of the birds sing around 20 sets of phrases for up to two minutes at a time.
He said "it is a major feat of coordination, especially when you consider (their) speed."
Both males and females hit their notes right on cue so that the ABCD phrasing flows along as though only one bird were singing.
Plain-tailed wrens are small, inconspicuous birds, save for their loud songs. The plain-tailed wren,
Thryothorus euophrys, has short wings and a thin, down-turned bill. It is not endangered and belongs to a New World bird family that contains 55 different species.
Male and female plain-tailed wrens all have their own repertoires of various song phrasings, but prefer to sing in coordinated groups.
"Sometimes one sex will sing a series of phrases without the other joining in," Slater said. "This was most striking in our group of seven, where there were only two males, and we often got B-D-B-D-B ... Apparently the poor guys couldn't keep up!"
Slater and his colleagues believe the synchronized singing is probably a form of defense against members of their own species.
"If you play a song from a speaker on their territory, the group will gather round singing like mad," he explained. "I imagine this is likely to be very intimidating to an intruding bird."
The singing might also help to synchronize breeding, he theorized. Most birds in temperate regions breed when day length increases in the spring.
On the equator, where these birds are, day length does not vary much. Instead of breeding all of the time, the birds seem to stay in synch with each other and "stimulate their reproductive systems" with the songs, not entirely unlike humans singing along to some sexy music on a hot date.
Daniel Mennill, a biologist at Canada's University of Windsor, studies a similar bird, the rufous-and-white wren. This species mostly sings duets, but the songs also are highly coordinated and seem to play an important role in territory defense and reproduction.
He told Animal Planet News, "The choruses of plain-tailed wrens astonish me."
Mennill added, "I agree that plain-tailed wren choruses may be the most highly coordinated and complex group vocalizations sung by any animal. As to whether they are more complex than human group vocalizations: I expect that the degree of coordination within plain-tailed wren choruses would be the envy of any choral conductor or orchestra leader."
Slater and his team next hope to analyze DNA from the wrens to determine how group members are related to one another.
