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Migratory songbirds may talk to one another more than we thought as they wing through the night. Each fall, hundreds of millions of birds from dozens of species co-migrate, some of them making dangerous journeys across continents. Come spring, they return home.
Scientists have long believed that these songbirds rely on instinct and experience alone to make the trek. But new research from a team of ornithologists at the University of Illinois suggests they may help one another out—even across species—through their nocturnal calls.
“They broadcast vocal pings into the sky, potentially sharing information about who they are and what lies ahead,” says ornithologist Benjamin Van Doren of the University of Illinois, Urbana-Champaign and a co-author of the study, published in Current Biology.
Birds of different species were flying in close proximity and calling to one another.
Using ground-based microphones across 26 sites in eastern North America, Van Doren and his team recorded over 18,300 hours of nocturnal flight calls from 27 different species of birds—brief, high-pitched vocalizations that some warblers, thrushes, and sparrows emit while flying. To process the enormous dataset of calls, they used machine-learning tools, including a customized version of Merlin, the Cornell Lab of Ornithology’s bird-call identification app.
The analysis revealed that birds of different species were flying in close proximity and calling to one another in repeated patterns that suggested a kind of code. Flight proximity was closest between migrating songbirds species that made similar calls in pitch and rhythm, traveled at similar speeds, and had similar wing shapes.
While the exact nature of their communication remains unclear, researchers speculate that the calls could relay details about landing sites, weather conditions, or individual traits like age or sex. Staying connected in this way might help the birds navigate more safely and effectively during the high-risk migration period to overcome challenges such as foggy conditions, inclement weather, or predator-rich landscapes.
“The function of these calls is still not quite fully understood,” Van Doren says. He does not deny that instinct plays an important role in navigation, he says. “But it seems that’s not the whole picture. Our work suggests that songbirds may also use social connections with other species during migration.”
Sharing migratory intelligence may be especially important for younger songbirds, who are just getting the hang of migration, says Van Doren. “Many of the birds migrating in the fall are juveniles,” he says, “and are navigating this dangerous journey for the first time.”
Van Doren and his team are eager to use machine learning and audio sensors to learn more about what specific kind of information birds are sharing. “We’d love to follow individual birds through their journeys using wearable microphones. That would help us disentangle the functions of these vocalizations and social connections,” he says.
“This paper is very exciting, and a huge feat,” says Tessa Alise Rhinehart, a biologist at the University of Pittsburgh who was not involved in the study and who specializes in using artificial intelligence to identify and interpret animal vocalizations. “Nocturnal flight calls are somewhat of an enigma to biologists and bird enthusiasts because they’re so difficult to study,” she continues. Just observing free-flying animals at night is a formidable challenge, she says, nevermind collecting data and running experiments.
“This is a careful, quantitative demonstration of that old saying,” says Dan Mennill, another biologist who studies migratory songbirds and was not involved with the study: “Birds of a feather flock together.” Especially when they are on the move. 
Lead image: Siarhei Tolak / Shutterstock
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