Two different Amazonian bird species sing the same song, say scientists.

Males of the two antbird species have evolved the same call to keep other males off their patch.

The researchers from Oxford University believe this is the first evidence of two separate species having evolved the same territorial song.

They report in the journal Evolution that this is an efficient way to compete for resources without risking injury through fighting.

Both birds, the Peruvian warbling antbird and the yellow-breasted warbling antbird, belong to the same large family.

"But the fact that two species have the same song is actually a pretty weird finding," said Joe Tobias from Oxford's Edward Grey Institute, who led the research.

The fact that the songs were "similar" had already been documented, which was why Dr Tobias and his colleagues started to look into the antbirds' biology.

"We thought they must be recently separated (from a common ancestor), but when we carried out genetic tests, we discovered that they both were ancient species - several millions of years old."

This finding made it even more strange that the birds should have such similar songs.

"So we decided to study them more intensively in the field - mapping their territories, seeing how they responded to the songs."