Crossing over

The team recorded the songs of both species and played the recordings to male birds that were resident in their own territory.

The birds were fooled by the imitation.

"When we played the song of the [rival] species, the resident bird responded as aggressively as it did to its own species," said Dr Tobias.

"It works - there's no overlap in the territory between the two species."

Dr Tobias explained that the males' song was a "territorial signal" that competitors recognised.

"That's why all the members of the same species tend have the same song - it's an easily recognisable deterrent signal," he said.

"In this case, the system seems to have crossed over the species barrier, and more than one species is using the same signal."

The scientists think that the birds' songs evolved over millions of years to "converge".

"We have more closely related species than these two in other parts of South America, and they have songs that are far more different, despite the fact that they have had less time to develop," said Dr Tobias.

Carlos Botero, an animal communication researcher from the National Evolutionary Synthesis Center in North Carolina, who was not involved in the study, described it as a "very interesting and well designed experiment".

"This is the first time this has been looked so carefully and in such detail," he told BBC News.

He pointed out that there were some differences between the antbirds' songs but that the playback experiment provided "very nice confirmation that the similarity between the songs was biologically relevant".

"The key thing is that the songs are much less different than other traits, such as their plumage, that have nothing to do with territory," Dr Botero explained.

The study brought to light, he said, that some signals, including birdsong, were driven by "social selection", where animals evolve to compete for the same resources.