One voice: Nest threats trigger united alarm signal in birds

Researchers have identified a unique vocal signal used by birds around the world to raise the alarm when a brood parasite enters their territory. Even more remarkable? Most of these birds have never met, and yet they’re all speaking the same “language”.

More than 20 species across Europe, Australia, Asia, and Africa were observed producing an eerily similar “whining” call when they spotted a brood parasitic bird — like the infamous cuckoo — lurking nearby.

The international team, including Dr Niki Teunissen and Professor Anne Peters from the Monash School of Biological Sciences, as well as William Feeney from Doñana Biological Station, Spain and James Kennerley from Cornell University, published its findings last week in the scientific journal Nature Ecology and Evolution, and provides new insights on the evolution of language and the learned responses of animals.

The research is one of the largest and most comprehensive studies involving brood parasites. The discovery has the potential to reshape our understanding of how animals learn to communicate.

Brood parasitic birds lay their eggs in other species' nests, forcing the host to raise the parasite’s young, often at the expense of their own offspring. By identifying the nest parasites on sight and vocalising their presence to others, the host species gain an advantage in preventing the parasites from laying eggs in their nests.

Experimental evidence in the field

To test why these different species, separated by oceans and up to 53 million years of evolution, all use the same call, the researchers used playback experiments to assess how the birds respond to unfamiliar whining calls compared to other vocalisations.

When the birds hear the recorded whining call playing, even if it is a different species from a different continent, they instinctively come to investigate what is happening.

The researchers also used model presentation experiments to assess how individuals responded to seeing a cuckoo compared to seeing predators and non-predators.

“Our Monash team contributed by undertaking experiments on purple-crowned fairy-wrens, a small endangered songbird that we have been studying for a long time at Australian Wildlife Conservancy’s Mornington Wildlife Sanctuary in the Kimberley, WA,” Niki Teunissen said.

“During the wet season, when the birds are breeding, we spent months in the field, watching and following birds every day to find their nests.

“Once the female laid eggs, we presented a model of a cuckoo, predator, or non-threatening control near the nest, and recorded the birds’ response.

“We found that like over 20 other species across the globe, the fairy-wrens produced whining calls when faced with a cuckoo near their nest.”

Gathering the required evidence involved researchers across multiple continents investing a substantial amount of time in the field to collect data, using similar methods, highlighting the collaborative nature of this large international study. Combining these efforts with the expertise of linguists, they were able to reveal the fascinating truth about this call: while birds instinctively approach when they hear the whining call, only when they come to investigate do they start absorbing the clues around them and learn when to produce the sound in the future.

The whining call thereby represents a mid-point between the innate (instinctive) vocalisations often seen in animals and fully learned vocal units such as words which make up human language.

Macroecological evidence

The researchers also looked at the geographic distribution of species that produce the whining vocalisations, to provide insight into its evolution. This revealed that species which produce the whining call tend to live in areas with complex networks of interactions between nest parasites and their hosts.

“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,” James Kennerley said.

The result is that the evolution of the whining vocalisation is affecting patterns of cooperative behaviours between birds around the world.

A language precursor

The findings challenge long-held assumptions about the sharp division between animal communication systems and human language, and suggest that learned communication systems, like human language, may have evolved through the gradual integration of instinctive and learned elements.

For the first time, this research 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 Darwin.

“It’s like seeing how evolution can enable species to give learned meanings to sounds,” William Feeney said.

The research was led by Doñana Biological Station in Spain, and Cornell University in the United States. It involved 30 researchers from more than a dozen institutions across the world, including the team from Monash University’s School of Biological Sciences.

This article was co-authored with Doñana Biological Station's William Feeney and Cornell University's James Kennerley.