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Right now, billions of birds are winging their way south as part of the annual winter migration—an event so enormous in scope and scale, it’s hard for humans to fully comprehend. But now, a new study provides a window into the lives of these animals like never before.
Using more than half a million records collected from five migration sites in the Northeast and Great Lakes regions, scientists have revealed that birds of different species form lasting relationships during migration.
What’s more, the researchers say these relationships could be ecologically meaningful, and potentially threatened by human-caused disturbances, such as climate change, according to a study published recently in Proceedings of the National Academy of Sciences.
While scientists have long suspected that there might be hidden connections between certain species at stopover points used during migration, the current study utilized records of 50 songbird species recorded over the course of 23 years of migration data to tease out a complex avian social network.
To study bird migrations, researchers often catch birds in nets and mark them with tiny, numbered leg bands at known stopover sites during the journey. Hints of songbirds’ social connections have emerged in some of these efforts.
For instance, each spring, American redstarts, magnolia warblers, and chestnut-sided warblers all get caught in the same sections of nets and within the same 20-to-45-minute windows of time.
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Similarly, when bird banders go out to these same sites in the fall, without fail, they will catch white-throated sparrows, ruby-crowned kinglets, and yellow-rumped warblers—again, in the same nets and at the same time. All of which suggests that these birds are not just randomly pausing their migrations when tired or hungry, but following repeatable patterns.
“It’s not easy to study migration and follow animals along their routes,” says Emily Cohen, a migration biologist at the University of Maryland Center for Environmental Science (UMCES) and an author of the study.
“But really, what you observe is all these species co-occurring. In the ocean, you have fishes and marine mammals along the same currents, and in the airspace, you have insects and birds and bats of all these species,” she says.
“In a way, it’s almost silly to think that they’re not interacting with each other,” says Cohen.
(Read more about the epic journeys of migratory birds.)
Do birds have buddies?
Interestingly, the current study doesn’t try to assess the quality of every interaction between the songbirds. Merely, it tracks which species are present at the same time, or on the flipside, which species have little to no overlap in an area.
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“With our data set, we can’t say whether these relationships are positive or negative,” says Joely DeSimone, who is also a migration biologist at UMCES, as well as the lead author of the study. “We could be seeing affiliations among birds that are chasing each other into the net, or we could be observing aggressive relationships.”
At the same time, though, the scientists found that songbirds were much more likely to show up together than they were to show signs of avoidance. In fact, out of all 50 species, the data revealed that only American redstarts and ruby-crowned kinglets appeared to be actively avoiding each other—for unclear reasons.
This social trend was a bit counterintuitive to the scientists, especially for closely-related species that overlapped in foraging behaviors. “We kind of were expecting to see competition among species that are eating similar foods,” says DeSimone.
Think about it: You have millions of animals weary from nonstop flights that can span thousands of miles. “They arrive in these habitats they’ve never seen before, essentially starving, and have to refuel, and rebuild their organs, rebuild their fat stores, and then continue on,” she says.
It would make sense if one bird looked at another as a competitor. However, the fact that so many species can be seen together, and so reliably, may hint at the beneficial nature of an avian social network.
“They also need to locate food quickly, and so the presence of other birds with similar foraging behavior or similar food preferences may signal to newcomers where the good habitat is,” says DeSimone.
For researchers, the next step is trying to figure out the exact nature of these songbird connections and what they mean for ecosystems, as climates change and bird resources shift.
(Learn more: How does bird migration work?)
You can be my wingman anytime
“One of the really remarkable things about this paper is that it’s looking at these huge, broad migrations across lots and lots of species,” says Janet Ng, a wildlife biologist with Environment and Climate Change Canada.
“There’s lots of research that looks at social relationships between individuals,” she says. “But this really allows for a big picture view of what’s happening.”
It also makes Ng wonder about the hidden relationships among other bird groups, and supports anecdotes among the shorebirds she studies.
In fact, some of Ng’s colleagues recently spotted a pair of semipalmated sandpipers standing together on a beach in Massachusetts this August. That in itself is not surprising, because semipalmated sandpipers travel thousands of miles each year in a migration that stretches from the Arctic all the way to South America.
What was mind-blowing, says Ng, was that this particular pair’s leg tags revealed that they’d been caught and banded at the exact same time, two years prior, in New Brunswick, Canada.
“Two years later, these birds were hanging out together again,” says Ng. “These birds migrated two cycles, and then were still observed together. So, it really poses a lot of questions.”
