An international group of researchers have discovered that birds on opposite sides of the planet produce almost identical ‘whining’ calls when beset by parasitic birds.

The study demonstrates the first known case of animal vocalisation learned from an innate response, indicating that natural selection may have influenced the origin of vocal language.

“It’s like seeing how evolution can enable species to give learned meanings to sounds,” says study co-lead, William Feeney from the Donana Biological Station in Spain.

The researchers observed how more than 20 different bird species from across 4 continents react to brood parasitism, making the study one of the largest and comprehensive on the topic so far.

Brood parasitism occurs when a bird lays their egg in the nest of another species, which forces the host to raise the young at the expense of their own offspring. While cuckoos and cowbirds are the most well-known species that practice brood parasitism, at least 300 species use the strategy in some way.

The birds included in the study were located throughout habitats in Australia, China and Zambia which meant they had no way of crossing paths with each other.

Nearly all the birds made a strikingly similar whining vocalisation when they found a parasitic bird in their territory. 

When other birds heard this whining call, they would come to investigate. In doing so, the authors suggest, these birds then pick up on this behaviour through a process called social transmission.

Eastern Phoebe nest with a brown-headed cowbird egg. Credit: Galawebdesign (CC BY 3.0)

“It’s then, when birds are absorbing the clues around them, that the bird learns when to produce the sound in the future,” says James Kennerley, a co-author of the study from the Cornell Lab of Ornithology.

“The fascinating thing about this call is that it represents a midpoint between the instinctive vocalisations we often see in animals and fully learned vocal units like human words,” adds Feeney.

The researchers found that the birds that produced these vocalisations tended to be from species that live in environments with strong networks of bird interactions, particularly between brood parasites and their hosts. 

“The evolution of the whining vocalisation is affecting patterns of cooperative behaviours between birds around the world,” says Kennerley.

“With birds working together to drive parasites away, communicating how and when to cooperate is really important, so this call is popping up in parts of the world where species are most affected by brood parasitism.”

For the authors, this vocalisation provides a potential insight into how human language may have evolved. They suggest that perhaps humans began to communicate from a similar spread of social transmission from instinctive vocalisations.

“For the first time, we’ve documented a vocalisation that has both learned and innate components, potentially showing how learned signals may have evolved from innate calls in a way first suggested by Charles Darwin,” Feeney said.

The findings from this study have been published in Nature Ecology & Evolution.