Songbirds, such as warblers, thrushes, orioles and sparrows, tend to migrate at night when the air is calmer, cooler and there are fewer predators lurking about. But the songbird flight paths may not be entirely instinctive, according to new research. Evidence from over 18,300 hours of recorded flight calls suggests songbirds may 'talk' to other species as they migrate, forming social connections and, perhaps, 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," said lead study author Benjamin Van Doren, assistant professor in the Department of Natural Resources and Environmental Sciences at the University of Illinois. "And we can certainly speculate that these flight calls could relate to navigation or finding suitable stopover habitat."
Research published last fall by co-authors Emily Cohen and Joely DeSimone at the University of Maryland Center for Environmental Science's (UMCES) 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. The team believes innate patterning and memory are still important drivers of migration behaviors, but their observations suggest science should view songbird migration through a social lens as well.
Over the years, there has been an increasing recognition of the importance of social information in bird migration, but scientists have documented this primarily in species that travel during the day, or in family groups. Similar to humans, young birds learn behaviors from observing other birds. But those visual cues go dark at night, when most songbirds travel, which prompted the team to wonder about other social cues. This study addressed those questions with acoustic recordings of autumn nocturnal bird migrations from 26 sites over three years in eastern North America.
The 18,300-hour acoustic record would have been a processing and analytical nightmare before artificial intelligence. Machine learning tools allowed the 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, they 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.
"This is an interesting result," said DeSimone. "It suggests that songbirds’ relationships reshuffle when they switch between flight and stopover, with in-flight relationships being related to flight behavior and navigation and terrestrial relationships being better explained by foraging behavior."
"Social interactions among migratory animals are poorly understood generally, despite the fact that they likely shape animals’ lives throughout their annual cycles, including during migration,” Cohen said. This study, she added, and the one from last fall "open the door to considering migrating animals as interacting communities during both passage flight and stopover periods, with many exciting discoveries on the topic likely to come in the future."
The team plans to follow up with more research, including attaching tiny microphones to individual birds and tracking their 'conversations' with flight partners throughout their migrations, but 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."
The study, entitled, "Social associations across species during nocturnal bird migration," is published in Current Biology [DOI:10.1016/j.cub.2024.12.033] and represents a collaboration between UMCES, the University of Illinois Urbana-Champaign, the Cornell Lab of Ornithology, the University of Leeds, Durham University, and Actions@EBMF.
The research was supported by an Amazon Cloud Credits for Research grant, the Cornell Lab of Ornithology, the Cornell Presidential Postdoctoral Fellowship, and the National Science Foundation [award no. 2146052]. Additional funding was provided by NERC (NE/V013483/1) and WildAI (C-2023-00057).
*Adapted with permission from the University of Illinois.
