Birds worldwide share evolved warning cry against parasites
An international team of researchers has discovered that birds across four continents produce nearly identical whining vocalizations to warn against brood parasites. This learned response builds on an innate sound, marking the first known example of such a hybrid vocalization in animals. The findings, published October 3 in Nature Ecology and Evolution, highlight how natural selection shapes cooperative communication.
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, according to an international team led by researchers at Cornell University and Donana Biological Station in Seville, Spain.
The study, one of the largest and most comprehensive on brood parasites, examined more than 20 different bird species across Australia, China, Zambia, and other regions. Brood parasitism involves birds like cuckoos laying eggs in other species' nests, forcing hosts to raise the intruders often at the expense of their own offspring. To counter this, hosts produce whining vocalizations upon spotting parasites.
"The results represent the first known example of an animal vocalization that is learned from an innate response shared across multiple species," the researchers noted. When a bird hears the innate whining sound, it instinctively investigates, then learns through social transmission to associate it with parasites.
"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," added William Feeney, co-lead and evolutionary ecologist at Donana Biological Station.
These species inhabit areas with complex interactions between parasites and hosts, where cooperation is crucial. "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," Kennerley explained.
The findings suggest this evolution influences global patterns of cooperative behaviors and challenges assumptions about animal versus human communication, potentially showing how learned signals evolved from innate calls, as Charles Darwin proposed.