A New Study Reveals Migration Isn’t a Solo Affair—It’s the Social Event of the Season

For songbirds, migration is the most difficult time of year. As they pass through thousands of miles of varied terrain, migrants must navigate a host of threats while urgently searching for habitat where they can rest and refuel. For a long time, it was assumed that birds completed these daunting journeys alone. But new research suggests migration is no solitary trek—it’s a social affair that encourages different species to band together.

In a pioneering study published last week in the Proceedings of the National Academy of Sciences, researchers discovered that songbirds socialize across species lines, forming “migrating communities” as they travel. While ornithologists previously believed migrant species like the American Redstart and Magnolia Warbler just happened to end up flying near one another, we now know that this closeness is no coincidence: It’s a sign that different bird species can form ecologically meaningful relationships and may even help each other out along their migratory journeys.

During peak migration, hundreds of millions of birds may be on the move in a single night. But there’s been a lack of research on how those travelers mingle, says the study’s lead author, Joely DeSimone, a postdoctoral researcher at the University of Maryland Center for Environmental Science. “It seems like common sense: When all of these birds are concentrating in really high densities, they are likely interacting with each other,” she says.

To test that hunch, DeSimone and her coauthors turned to the vast trove of data collected at bird banding stations: “It seemed like a gold mine,” she says. At these sites, professional banders capture birds in mist nets, quickly record as much information as possible—age, sex, weight, and a host of other measurements—before fitting the birds with unique identification bands and releasing them back into the wild. After gathering decades’ worth of data from five such stations in eastern North America, the researchers ended up with over 500,000 records representing 50 bird species.

The scientists started simple with their analysis, first noting when two species of migrants ended up in the same net at around the same time. Next, they had to tease out whether these pairings were more than chance encounters. DeSimone already knew that birds with similar habitats, foraging styles, and migratory pathways, such as Ruby-crowned Kinglets and White-throated Sparrows, are generally more likely to occur near one another—so she designed statistical tests that accounted for these kinds of overlaps. If the tests still revealed that two species were showing up together more often than expected, it would be a sign that they were interacting in some way.

The results proved just that: Migrants appear to have their own interspecies social networks. Not only did the authors find clear links between some pairs of species, they also observed that the same pairings showed up across banding stations. That consistency across different locations suggests these interactions play a significant role in a species’ migratory ecology.

Before examining the results, DeSimone had expected that relationships between migrants might be tense. Since previous research indicated that the hungry travelers might come into conflict as they vied for limited food and habitat, she wondered if species with similar diets would avoid one another and appear in the same net infrequently. Instead, the results showed that birds like Nashville and Tennessee Warblers seemed to seek each other out during migration. Species with similar foraging styles and genetic relationships may learn from one another and follow their traveling buddies to patches of good habitat, boosting their odds of a successful migration, the authors suggest.

DeSimone stresses that we don’t know exactly what these interactions between species might look like—it’s still possible the linked species were competing and chased one another into a bander’s net, for instance. Understanding how migrants socialize will require carefully observing them along their routes, something birders may be well-positioned to help with. “I would love more birder interpretations of my paper and what they see out in the field,” DeSimone added.

For her part, DeSimone is already planning a follow-up study to examine how relationships between species are shifting as climate change alters their migration timing. She will also look at interactions between migrants and resident species; many birders have seen in-transit warblers, for example, mixed in with local chickadees and titmice.

Already, DeSimone’s work marks an exciting shift in our understanding of migration, says Jill Deppe, senior director of Audubon’s Migratory Bird Initiative. “For a long time, scientists have been working under the idea that a lot of these birds just sort of do their own thing during migration,” she says. “Because we weren’t sure about whether birds were moving together and had these interactions, a lot of our approach to conservation has been one species at a time.”

To now have such strong proof of cross-species relationships is a breakthrough, Deppe says, and one that could have big implications for protecting migrants, which represent over 80 percent of the estimated 3 billion birds lost since 1970. By switching to a multi-species approach, conservationists can focus their efforts where they’ll have the greatest impact. “It gives me hope that we’re going to advance our conservation actions at a pace that’s warranted by the urgency of the declines we’ve seen,” she says. “One species at a time just isn’t going to be fast enough to protect these species and bend that bird curve.”