It takes two to duet  , and one question for scientists is how these coordinated   performances arise -- in birds. Are they the result of cooperation, a way in which one pair signals to others that they've got it together? Or are they the result of conflict, evolving to avoid one partner's song interfering with the other's? A study of duetting in Peruvian warbling antbirds  suggests that it might be a little of both, and that context is everything. Joseph A. Tobias and Nathalie Seddon of the University of Oxford show in Current Biology that sexual conflict can cause the female of a pair that normally cooperates to "jam" the male's song by singing over it. The researchers exposed antbird pairs to recorded songs of other antbirds and monitored the songs the pairs produced. In one experiment, they played the songs of an intruding pair. In this case, the resident pair "both are likely to lose their territory, so both should cooperate," Dr. Tobias said. And they do. They produce a coordinated duet that in effect tells the intruders to keep away. But when the researchers played the song of a single female, the pair behaved differently. "You'd expect the resident female to be highly motivated to defend her position in the partnership," Dr. Tobias said. And that's what occurs. The male sings its heart out, flirting  with the single female, and the female of the pair does its best to interfere with the song by singing over it, apparently to make her mate less attractive to the other female. "It's clear that the male doesn't like what she's doing," Dr. Tobias said. The behavior "breaks up what is otherwise a very cooperative situation into a more complicated signal," he added. It's the first evidence of this kind of signal jamming among pairs, Dr. Tobias said. And in that it leads the male to alter its song to avoid the female's interfering notes, it shows that this kind of conflict could, over a long period, drive the evolution of coordinated song. How did the researchers conduct the experiment?
They put the antbird pairs back to nature and observe them. They played different recorded songs of other antbirds. They put an antbird to the other's territory and observed. They played the songs of an intruding pair.