Imitate To Communicate: Even Singers In The Bird World Have To Deal With Cover Artists

A team, led by Dr Joseph Tobias and Dr Nathalie Seddon from the Edward Grey Institute, University of Oxford, analysed the calls and songs of two antbird species who live side-by-side in the Amazon rainforest; the Peruvian warbling-antbird and the yellow-breasted warbling-antbird.

The main aim of the study was to investigate their similar songs, and in particular to test the theory that songs can become increasingly similar to enable effective communication between competing species. This notion is controversial as many scientists argue that convergence in territorial or mating signals results in needless confrontation or crossbreeding and the creation of hybrids.

"Biologists have long been fascinated by convergence in ecological traits as it offers tangible evidence of evolution and the forces of selection by which it operates, but until now there is no clear evidence that social competition between animal species can produce convergent signals" said Tobias. "We examined this idea by analysing the structure and function of songs in two birds which we knew to be strong social competitors."

The team studied these species in Peru and Bolivia at one site where they lived together, and two sites where they lived in isolation. First, they recorded three sets of signals; songs, calls, and plumage colour of both species (including a total of 504 songs from 150 individuals). Then, to test the significance of songs of both types, they played them back to individuals of each species.

The results showed that territorial songs of both species were extremely similar particularly where they lived together, such that territorial birds treated songs of both species as equally threatening. Meanwhile, non-territorial signals like calls and plumage were highly divergent.

"In effect, the territorial songs of these birds are more or less interchangeable in design and function" said Tobias. "Given that they last shared a common ancestor more than 3 million years ago, it is almost equivalent to humans and chimpanzees - which diverged around 5 million years ago - using the same language to settle disputes over resources."

"Our results provide the first compelling evidence that social interaction can cause convergent evolution in species competing for space and resources," concluded Tobias."They also suggest that while competition drives convergence in territorial songs, this is offset by divergence in non-competitive signals such as plumage colour to promote species recognition and reduce the chance of interbreeding."