The night sky teems with migrating songbirds, aloft in their millions following routes etched in evolutionary time. But those flight paths may not be entirely innate, according to new research led by the University of Illinois Urbana-Champaign. Evidence from over 18,300 hours of recorded flight calls suggests songbirds may “talk” to other species as they migrate, forming social connections and — just maybe — exchanging information about the journey.

“We can’t be sure what they’re saying, but birds might broadcast calls during flight to signal their species, age, and sex. And we can certainly speculate that these flight calls could relate to navigation or finding suitable stopover habitat,” said lead study author Benjamin Van Doren, assistant professor in the Department of Natural Resources and Environmental Sciences, part of the College of Agricultural, Consumer and Environmental Sciences at Illinois. Van Doren began this research at the Cornell Lab of Ornithology.

Work from last year by the study’s co-authors at the University of Maryland Center for Environmental Science, Appalachian Laboratory suggested birds “buddy up” with other species at stopover sites during migration, but there was no evidence until now that different songbird species pair up or communicate vocally on the wing. Although Van Doren believes innate patterning and memory are still important drivers of migration behaviors, he says it’s time to rethink songbird migration through a social lens.

“In recent years, there has been an increasing recognition of the importance of social information in bird migration, but scientists have mainly documented this in species that travel during the day or in family groups,” he said. “The social environment also seems to be important in species like hawks and storks that form huge aggregations during their daytime migrations. Young birds learn behaviors from observing other birds and how they navigate — and not necessarily from family.” 

But those visual cues go dark at night, when most songbirds travel. That’s what led Van Doren to wonder about other social cues. Fortunately, he had access to acoustic recordings of autumn nocturnal bird migrations from 26 sites over three years in eastern North America. 

“These nocturnal acoustic recordings are really the only window onto this unseen but absolutely massive flow of birds — hundreds of millions aloft over the U.S. on any given night during migration,” Van Doren said. “It’s something people aren’t usually aware of because it happens when we’re sleeping.”

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The 18,300-hour acoustic record would have been a processing and analytical nightmare before AI. Now, a machine learning tool allowed Van Doren’s team to quickly detect the signature flight calls of 27 species, including 25 well-sampled songbirds.

After identifying species, the team measured how often certain calls co-occurred in time, testing intervals of 15, 30, and 60 seconds. Regardless of the time interval, they found stronger associations between species than expected by chance alone.

Looking to explain these associations, the found species’ wing lengths and the similarity of their calls were the most important factors. In contrast, birds that “buddy up” during stopovers weren’t maintaining those relationships in the air, and they weren’t necessarily flying with closely related species or birds that shared their preferences for specific habitats.

“Species with similar wing sizes were more likely to associate, and wing length is directly linked to flight speed. If you imagine two species flying at similar speeds because they have similar wings, then it’s much easier for them to stick together,” Van Doren said. “As for vocalizations, it is possible that species’ calls have converged over time because of this social link or that species that happen to give similar calls are simply more likely to gravitate towards each other.”

Van Doren notes that 25 is a small subset of the songbird species migrating at night, some of which don’t vocalize at all during flight. He and his team plan to follow up with more research, including attaching tiny microphones to individual birds and tracking their “conversations” with flight partners throughout their migrations. 

Still, these preliminary results raise many intriguing, if speculative, notions. For example, short-lived songbird species who can’t rely on their parents to show them the way may instead rely on social ties with others to make the trek. Also, the precipitous loss of bird biodiversity with climate change and habitat loss may jeopardize partner species that co-migrate. 

“This study really calls into question the long-held idea that songbirds migrate alone, solely following their own instincts,” Van Doren said. “Learning more about the consequences of these social connections — not only for migration, but also for other aspects of their biology — will be important to inform and manage the risks they face in a changing world.”

IMAGE CREDIT: Andrew Dreelin

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