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Birds separated by vast geographic distances and millions of years of evolution share a remarkably similar learned vocal warning to identify parasitic enemies near their nests, an international team of researchers has found.

The results represent the first known example of an animal vocalization that is learned from an innate response shared across multiple species.

The findings, published in Nature Ecology & Evolution, provide a glimpse into the role natural selection can play in the evolution of vocal communication systems.

The study, led by researchers at Cornell University and Donana Biological Station in Seville, Spain, is one of the largest and most comprehensive studies concerning brood parasites to date.

Brood parasitism occurs when birds, such as cuckoos, lay their eggs in other species' nests, forcing the host to raise their young, often at the expense of the host's own offspring. That's why it is advantageous for the host species to identify and try to prevent nest parasites from laying eggs.

The researchers found that more than 20 different bird species across four continents produce nearly identical "whining" vocalizations when they spot a parasitic bird in their territory.

The researchers wondered why birds from locations spanning Australia, China and Zambia all use the same call to identify their parasites, despite never coming into contact with each other.

When a bird hears the warning call, it instinctively comes to investigate. That's when, according to the researchers, the birds start absorbing the cues around them—what Damián Blasi, co-author of the study and a language scientist at Pompeu Fabra University, Spain, calls social transmission.

"It's then, when birds are absorbing the clues around them, that the bird learns when to produce the sound in the future," said James Kennerley, co-lead author and postdoctoral fellow at the Cornell Lab of Ornithology.

"The fascinating thing about this call is that it represents a midpoint between the instinctive vocalizations we often see in animals and fully learned vocal units like human words," said William Feeney, an evolutionary ecologist at Donana Biological Station in Seville, Spain, and co-lead of the study.

The research also revealed species that produce the whining call tend to live in areas with complex networks of interactions between brood 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," said Kennerley.

The result, he said, "is that the evolution of the whining vocalization is affecting patterns of cooperative behaviors between birds around the world."

The link between the innate whining sound and the learned response by the bird is what makes this study unique, the authors said.

"For the first time, we've documented a vocalization 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. "It's like seeing how evolution can enable species to give learned meanings to sounds."

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