In Wytham Woods near Oxford, a small green-and-yellow bird lands at a feeder. Its mate from the previous breeding season arrives shortly after. But instead of feeding together, the two keep their distance. By spring, they’ll have split.

This kind of separation isn’t unusual. A new study from the University of Oxford and the University of Leeds finds that great tits often show signs of “divorce” during the winter, well before the next breeding season.

“Our results show that bird relationships are far from static,” said Adelaide Daisy Abraham, the study’s lead author and a PhD student at Oxford. “We found a clear behavioral signature in the winter months that can forecast a pair’s likelihood of divorcing by spring.”

The research, published in Proceedings of the Royal Society B, uses data from one of the world’s longest-running wild bird studies.

The Subtle Science of Avian Separation

Great tits (Parus major) are a common European songbird—small, colorful, and surprisingly devoted. Each spring, they form monogamous pairs to breed. Males feed females during incubation. After the chicks hatch, both parents join in the scramble to feed their young with caterpillars.

By mid-summer, though, the work is done. The chicks fledge. The woods go quiet.

Then comes winter. The birds stop breeding, but they don’t vanish. They form loose, ever-shifting flocks as they forage. Hidden among the branches, dramas unfold—some birds stick close to their former partners; others drift away.

To understand these winter dynamics, Abraham and her colleagues turned to technology. Each bird in the study was outfitted with a tiny electronic tag. These tags registered each visit to an array of feeders scattered throughout Wytham Woods. The data allowed scientists to track not only individual birds, but who they were spending time with—and when.

Some pairs, the researchers found, continued to feed together throughout the winter. Others began visiting at different times. The digital record revealed a slow-motion uncoupling.

“Those divorcing birds, they, from the start, are already not associating as much [at the feeders] as the faithful birds,” Abraham told NPR. “That only increases as the winter goes on.”

A Bird’s Eye View of Relationship Decline

The researchers identified four kinds of couples: “faithful” pairs that stayed together across consecutive breeding seasons, “divorcing” pairs that split before the next season, “new” pairs forming for the first time, and “juvenile” pairs just entering adulthood.

Of these, divorcing pairs stood out in every behavioral measure.

They had the lowest “winter association scores”—a metric of how often they appeared in the same flocking events. They were also far less likely to list each other as their closest social partner, and they rarely fed at the same time. Over the course of the winter, these trends sharpened.

In contrast, faithful pairs became even more synchronized. They spent more time flocking together and were increasingly likely to feed in close succession, even within three seconds of each other—a metric the study called “visit adjacency.”

By late winter, the divide was stark. Faithful birds acted like a team. Divorcing birds didn’t.

And these patterns didn’t just apply to the pair’s overall behavior. The difference held even when researchers compared how individuals acted with their mates versus how they acted with other birds. That is, divorcing birds were not simply less social—they were selectively distancing themselves from their partners.

“This work is an important step towards uncovering the social mechanics behind pair bonding and fidelity in the wild,” said Professor Ben Sheldon, head of the Wytham Woods study.

Wild Bird Drama?

The idea of birds divorcing might seem like anthropomorphism. But researchers are careful to clarify: this is not about court proceedings in the treetops.

“These birds aren’t really getting divorced,” Abraham said. “They’re not serving each other with papers, or appearing in tiny courtrooms high in the trees.”

Still, the term “divorce” is used in scientific literature to describe monogamous bird pairs that separate despite both partners being alive and capable of breeding. And in great tits, divorce isn’t rare.

Why it happens remains a mystery. Previous studies have linked separation to poor breeding success, suggesting some birds may abandon unsuccessful partners to try their luck elsewhere. But this new study adds a fresh dimension: timing.

Rather than splitting up abruptly in spring, many birds appear to begin the process months earlier. Divorce, the authors argue, is not an event—it’s a slow unfolding.

The distinction between faithful and divorcing pairs emerged “from the very beginning of the winter period,” the researchers wrote. This suggests that birds may be responding to information gathered during the previous breeding season—or that certain birds are predisposed to maintain weaker bonds.

Intriguingly, newly forming pairs at first resembled divorcing pairs in their behavior, but gradually started acting more like faithful ones. By late winter, these new couples showed increasing closeness.

The Broader Implications

What makes this study especially significant is the scale and precision of its data. By tracking individual birds across multiple years, the team could link relationship dynamics to real-world behavior in the field.

“Following these individual birds across seasons and over many years allows us to see how relationships form and break down in nature in a way that short-term studies wouldn’t,” said Dr. Josh Firth, a senior author from the University of Leeds.

Beyond birds, the findings hint at broader principles of animal behavior. If the social signals of divorce can be predicted in great tits, might the same apply to other species—mammals, fish, even humans?

The idea that social behavior in one season shapes outcomes in another isn’t new. But this study shows that even as birds navigate the flux of winter flocks, they are making decisions that shape their future.

“There is actually a lot more going on in those flocks of birds out your window than you think there is,” Abraham said.