A study of rufous-and-white wrens (Thryophilus rufalbus) in Costa Rica has shown that male and female duets function as a way of warding off rivals that might encroach on their territory. The wrens were recorded with eight recording microphones hooked to a single laptop, so that the computer could track the birds' relative positions to each other while they sang.
"Your first impression after you hear the duet of a pair of tropical birds is one of great harmony and cooperation," said Daniel Mennill of the University of Windsor. "Their duets require coordination and synchronization, and my multi-microphone recordings confirm that birds do coordinate their activities by performing duets. But there is a darker side to duetting; tropical birds also perform duets in very aggressive contexts, and respond with special aggression to rival individuals of the same sex. Their voices are beautiful harmonies, but they're also aggressive audio warfare."This particular finding does not surprise me at all. When I listen to Carolina wrens (Thryothorus ludovicianus) performing, I often hear a male singing and a female trilling in one position while a male sings (and sometimes a female trills) from another direction. It has often struck me that they are probably cooperating in their territorial defense, so it is neat to see that type of behavior confirmed in other wrens by more sophisticated methods.
The researchers found that male and female wrens approach each other following duets and use them to play a version of the children's game Marco Polo. "One bird sings, listens for the song of its partner, and moves towards their partner after hearing a response," Mennill said.
In another set of experiments, Mennill used two loudspeakers to simulate the voices of a pair of duetting wrens and found that birds fight duets with more duets. As soon as the birds heard the duets of a rival pair, their singing rate "shot through the roof," he said, evidence that the melodies play an important role in aggressive territory defense.
You may have heard the name of the lead researcher before; he is part of the team looking for ivory-billed woodpeckers in the Florida panhandle. Birder's World posted an interview with him about his research in Costa Rica and Florida on their blog.