Twice a year, birds as small as hummingbirds and as large as cranes make an immense journey from one habitat to another, fulfilling the age-old compulsion to migrate. Oftentimes, you hear them passing overhead before you see them because of the loud squawks they make. The mysteries of this mass movement have baffled humanity for millennia. How do they navigate at night (the time when most distances are covered)? What makes them decide when and where to stop to rest and feed?

Thanks to advances in technology—including better devices to record sound and artificial intelligence to process data—scientists think they have the answer: Birds talk to each other. Their kind of "talk" isn't like ours of course, but it is communication nonetheless that helps them take the journey together.

In their Current Biology paper this month, ornithologist Benjamin Van Doren and his team at the University of Illinois at Urbana-Champaign report on the social associations between bird species during nocturnal migration flights on the Atlantic flyway. Using vocalizations (“flight calls”), 17 out of 27 songbird species were found to communicate the most. Flight call exchanges predominated within bird families, e.g., New World warblers, though calling between families occurred as well. Between-family associations appeared just as strong as those within families. The main requirements seemed to be similarity in wing lengths (which determine flight speed) and in vocalizations, which typically consist of short, species-specific calls lasting about a tenth of a second.

Up to now, migrants were believed to travel “alone and without the aid of other individuals,” Van Doren said over Zoom. Juveniles, those hatched a few months prior to fall migration, must follow innate and experiential knowledge or “endogenous programming” when making decisions, like timing and choosing direction. But they are too young to have much in the way of experience. Therefore, migrating in hearing range of other, often older birds must be an advantage. This research provides the first quantitative evidence that social information can supplement endogenous programming.

The team recorded more than 18,000 hours of nocturnal bird migration. Using AI, they were able to gather and process more than 175,000 in-flight vocalizations of 27 species of North American birds.

“Our work has shown that there is potential for more of a social component to migration,” than previously understood, Van Doren said. “It’s not these birds just following their instincts across the hemisphere,” he added, “but that they may actually be connecting with other individuals and species along the way, and maybe that helps them complete these mind-bending journeys.”

Wing length and type of vocalization obviously apply to members of one’s own species but overlap with others as well. Think hundreds of sparrows or thrushes flying at the same time as tanagers and warblers, many of which vocalize throughout the flight period. They appear to be helping each other, and, in turn, being helped, in a kind of agreement.

The benefits are clear. “Migration is a dangerous time,” Van Doren said. “These birds, many on their first migration, have to cross these unfamiliar areas, find enough food to survive, and reach their destinations. And we can imagine that staying in touch with other individuals could certainly help them accomplish these goals by, say, sharing navigation information or helping them get through tricky conditions (like inclement weather), or teaming up in preparation for landing.”

It has long been known that storks and other large migrants communicate en route. This study confirms that smaller species also exchange information during flight.

Birdwatchers will recognize comparable collective behavior in their own backyards and feeders. Bark foragers like nuthatches, chickadees, and creepers will search for grubs on the same tree at the same time. Ground-feeding species like grackles and robins also search together in loose groups. These mixed flocks increase their chance of finding new sources of food and lower the odds of being surprised by a predator.

Van Doren mentioned another flight-call study from 2019. It found that the number of bird strikes on a well-lit, low-rise building in Chicago correlated with the number of flight calls recorded. The reason for this is not yet known. The researchers suggest, however, that the flight-call information actually lures more birds into the light. When an age-old adaptation has to deal with the built environment, there can be lethal results.

Further declines in bird populations (by one estimate, nearly 3 billion have already been lost since 1970) will affect flight calls. As the densities of flocking migrants drop, their collective ability to navigate properly could go down as well. Greater risk of collisions, and unsuitable stopovers will put greater pressure on already-threatened migrating species, with consequences for entire ecosystems. These acoustic studies provide conservation biologists with another tool in the fight to save them.