Keep out! Rival males of these two South American antbird species--the yellow-breasted antbird (left) and the Peruvian antbird--evolved to sing the same song.

Joe Tobias

Two Bird Species Sing as One

Talk about a common tongue. Even though two species of South American antbirds have been evolving independently for more than 3 million years, they sing nearly identical territorial songs. "It's almost the equivalent of humans and chimpanzees using the same language to settle disputes over resources," says Joseph Tobias, an ornithologist at the University of Oxford in the U.K. But rather than causing confusion, the identical songs actually serve a valuable purpose.

One wouldn't expect the two antbirds to have much in common. Although they both live in the southwestern Amazon, one species, the yellow-breasted antbird (Hypocnemis subflava), prefers bamboo patches whereas the other, the Peruvian antbird (H. peruviana), likes tall and dense forests. The birds also look different: H. subflava males sport yellow chests and buff-colored flanks, for example, whereas the males of H. peruviana have white chests and reddish-brown flanks. Yet the songs the males of each species use to defend their turf are indistinguishable to people: In previous studies, researchers showed that neither human ears listening to the songs nor human eyes studying spectrograms of the songs could identify any notable differences. (Hear both songs back-to-back here.)

But do the songs sound the same to the birds themselves? To find out, Tobias and Oxford ornithologist Nathalie Seddon recorded 504 songs from 150 birds of both species. They also recorded the territorial songs of other closely related antbird species. The duo then played the recordings to individuals from both species and sat back to watch.

Males reacted the same way to songs from either H. subflava or H. peruviana. At the first note from a rival, the territory holders rushed in, "ready for a fight," says Tobias. "They weren't scared and were very aggressive." But the same males barely acknowledged the territorial tunes of other antbird species, even those more closely related to themselves.

Tobias says the two birds probably evolved the same territorial songs because their preferred habitats often overlap, such as in mixed areas of forest and bamboo. Because the males in these two species are competing for the same real estate with males of their own kind as well as with males from the neighboring species, they simply use the same song to deter both. "It's more efficient," says Tobias. "You would never find such convergence in mating songs," he adds, because male birds of unrelated species aren't competing for females. The team reports its findings in the current issue of Evolution.

The study offers a "convincing case" of convergent evolution, says Darren Irwin, an evolutionary biologist at the University of British Columbia in Vancouver, Canada, although in this instance, he says, species are becoming more alike despite the fact that they are fighting over the same resources. This kind of convergence, though rare and controversial, "likely occurs more often than commonly thought," he says. Adds J. Albert Uy, an ornithologist at Syracuse University in New York, "The study should revive interest" in this more unusual evolutionary force.