Bare trees, the smell of dead leaves, silence, mud. The winter woods in Oxfordshire are lovely, if you like that sort of thing. If you’re a scientist with the legendary Wytham Tit Project, it doesn’t matter whether you like it or not. The batteries in your trick bird feeder must be changed every two days or your data will be ruined. And getting the most and the best data is the name of the game here in Wytham Woods.
So Oxford University graduate student Friederike “Freddy” Hillemann loads 50 pounds of rechargeable batteries into her pack and trudges through the bramble and bracken. We’re about 100 feet from a quartet of tube-shaped bird feeders when a spiffy little round green-and-yellow bird, a Great Tit, dives off the first tube. It looks like a chickadee that got lost in a watercolor kit. As we get closer, a smaller and drabber featherball called a Marsh Tit lands on the third feeder, which goes click click click.
Hillemann stops short. “Did you hear that clicking sound? That’s the feeder opening.” It’s the sound of success. Her gizmos, as intended, are spying on birds’ social habits.
Every fool knows that birds of a feather flock together, but it takes a scientist to find out why. With experiments like this one, Hillemann and a dozen others at the Edward Grey Institute of Field Ornithology (EGI) at the University of Oxford are probing what it means for a bird to have friends, and how those friendships shape life and death in the woods.
This particular experiment relies on “selective feeders” outfitted with circuit boards and programmed to open only for specific birds. When that tit alighted on that feeder, an antenna on the tube pinged a tiny chip in a band on its leg. A unique ID number identified it as a Marsh Tit. Hillemann has programmed this feeder to accept Marsh Tits, so the door slid open, allowing the bird to grab a seed. In this quartet, two other feeders are also species-specific; one opens only for Great Tits, the other only for Blue Tits. (Wytham is a very titty place; it also has Coal Tits and Long-tailed Tits, but they’re not part of this study.) The fourth feeder opens for all three species, and it’s the most popular one. Apparently, birds prefer to eat alongside other species, even if, like the Marsh Tit, they’re intimidated by bigger birds. Hillemann is trying to find out why.
Like that Marsh Tit, almost every Great Tit, Blue Tit, and nuthatch in Wytham Woods has been caught, banded, and digitally barcoded with a chip. As long as Hillemann keeps the batteries charged, every trip that every banded bird makes to an experimental feeder is automatically recorded. The result is big data—astronomical by the standards of fieldwork. By the end of this study in March, she will have hundreds of thousands of data points from about 500 birds.
Even so, the data from Hillemann’s project is a drop in the bucket on the scale of the Wytham Tit Project, the world’s longest-running continuous study of individual avian lives. Its database now tops 34 million records from 10,000 individual birds. Never before have researchers had such granular information about the social lives of so many generations of wild animals on such a large scale.
The engineers at Facebook would be envious of the information that Hillemann and her colleagues vacuum up about the birds’ lives: Who hangs out with whom and how often, who’s dating whom (and who is cheating), where and how often they have lunch. The size of this dataset permits the Oxford team to apply social network theory and machine learning, powerful analytical tools that are widely used to understand and predict human behavior in social media.
What’s emerging is the revelation that for flocking birds, social connections are a matter of survival. They influence who finds food, who gets the best territories, who gets sick with parasites or infections. By studying avian social rules, the scientists at EGI hope to predict resilience—how species will manage climate change and other monumental threats, and how we might help them do it.
Busy talking to me about all these ideas, Hillemann hooks the fresh battery up to the wrong terminals on her unit, zapping a fuse on the circuit board that controls the door. As she tinkers with a replacement, I try not to distract her. I glance up as a small band of tits darts through the trees. At this point, I no longer see birds. I see little packets of data, bursts of mobile social information. This isn’t just a damp gray forest. It’s the social network, for birds.
In 1945, David Lack, an expert on the quintessentially British subject of European Robins, became director of EGI at Oxford. Initially, he planned to continue his robinnical studies. Then he visited a scientist in the Netherlands who was tracking Great Tits.
Lack was immediately struck by how much easier they are to study. Tits, close relatives of North American chickadees and titmice, are tough little birds that don’t stress out when they’re caught and banded. They are plentiful in Oxfordshire. The tits there don’t migrate. Best of all, they readily use scientist-provided nest boxes. “They find our nests, so we don’t have to find theirs,” says emeritus fellow Chris Perrins, who began working with Lack on the Oxford tits in 1957.
Even back then, the Wytham Tit Project was about data. Lack and Perrins asked simple questions that turned out to be hard to answer: How do birds decide when to breed? Why do some pairs lay lots of eggs and others just a few? The way to address these queries was to track many birds over many generations.
So the Great Tit became the lab rat of ecology: a convenient, easy-to-study species that stands in for woodland songbirds in general. Each generation of Oxford ecologists who keeps the study going inherits a steadily growing treasure trove of data—as well as lives ruled by muddy boots, cold fingers, and sleep deprivation.
On a still, cold February morning, eight young Oxford researchers repeat a ritual that has played out every week in winter for more than 71 years—what they call ringing, and Americans call banding. Frost crunching underfoot, they unspool a mist net in front of a row of sunflower-seed feeders. “Good ringing weather,” says field technician Keith McMahon, who has spent five winters in Wytham. He’s right. Within minutes, 10 Great and Blue Tits are quivering in the net.
They pluck birds from the fine mesh like plums off a tree, then carefully unravel each fluffball and pop it into its own small linen bag, which on this cold day is pre-warmed in a pocket. There is no chitchat, just the indignant wheezes of the birds.
The researchers quickly weigh and measure the little blue-and-green jobs. They clip regular bands, plus rings with the electronic ID chips, around their legs. While he works, Josh Firth, a junior research fellow at Oxford’s Merton College, is simultaneously coaching the newbies, cracking jokes, and teasing McMahon. Firth tucks a Blue Tit headfirst into a cylindrical film canister. He sets it on the scale: less than a dozen grams, and 100 percent feisty. When he takes it out of the canister, it bites him, and as he hands it over to me to release, it transfers its fury to me. (Note: Blue Tit bites do not hurt.) I admire its fluffy little face, then open my hand. The bird zooms away, its slate-blue back vanishing against the winter sky.
Thanks to these banding parties, the scientists know the ancestry of almost all the Great Tits born in these woods. Some family trees go back 35 generations, the equivalent to tracking humans over a millennium. “As many scientific projects do, it generated question after question,” says Ben Sheldon, who joined the Wytham Tit Project in 2000 and now heads EGI and the Oxford department of zoology. It also generated answers. For instance, the project was one of the first anywhere to prove that birds had begun laying their eggs earlier in the year, apparently in response to climate change. (Great Tits now lay nearly three weeks earlier than in 1960.)
To observe what fledglings do after leaving the nest, in 2007 Sheldon began attaching passive integrated transponder tags to bird bands. Battery-free chips, they rely on the cheap radio-frequency identification technology that is ubiquitous in trackers in your passport and books from your library. By the winter of 2011, his group had 65 RFID-equipped feeders around Wytham Woods, a comprehensive tit spy network. Each visit by every tagged bird would create a unique record including identity, time, and place, creating a map of its daily movements. The Wytham Woods became an ornithological panopticon, all-seeing and all-knowing, following birds from birth until death.
In those data, Sheldon noticed something surprising: Birds rarely arrived alone. They came in groups, and the same groups appeared at different feeders. They seemed to be friends. “We realized there was also lots of social information here,” he says. “That, for me, was really exciting.” It suggested yet another deceptively simple question: Do wild birds have friends? If so, why? This surveillance system allowed the researchers to monitor relationships between free-living wild animals at scale and connect those patterns with the birds’ pedigrees to explore how genes shape social behavior.
To quantify and track relationships, Sheldon turned to social network analysis. First articulated in the 1930s, these ideas now surge with new energy as companies tap into them to make sense of social media data from the likes of Twitter and LinkedIn. The methods offer ways to interpret social behavior on the smallest and largest scales, from interaction patterns between two individuals to the consequences of links in an interconnected social group. With wild animals, social network analysis had primarily been confined to computer simulations. But at Wytham, it would finally be possible to see what the birds were really doing.
We’ve banded, recorded, and released the first batch of tits, and already 22 more are hanging in the net. Considering their predicament, they seem oddly calm. It strikes me that most of these birds probably know each other. They’re trapped, but they’re among friends.
I try to picture the invisible relationships between them, the lines that connect the friends, friends-of-friends, and so forth, forming a dense knot of nodes and links. Radically simplified, that’s the gist of this approach: Six degrees of separation, with tits.
Social network analysis offers ways to think about systems. In a population of dozens or thousands, you can detect and weigh all the specific relationships between pairs of individuals, then sum them all together to describe the group as a whole. On the finest level, you can see which individuals are the social hubs or single out the connector individuals that bridge otherwise un-related groups—the Kevin Bacons of tits. On a bigger scale, you can measure whether a group is tight-knit, with each individual having many strong links to others, or loose, with each member having only a few friends.
At first Sheldon and his team simply tried to understand the basic features of these networks. In winter, they learned, Great Tits form little groups that on average include between four and eight birds. The groups are fluid and dynamic; on any one day, a bird might belong to more than one such flocklet. Two bird friends generally spend between 5 and 25 percent of their time together; an average Great Tit has about 50 friends over the course of the winter.
One early finding, led by then graduate student Lucy Aplin, was that a bird’s place in its flock is partly determined by personality. All tits are brash and inquisitive, but some are gutsier than others. Those bolder birds have many social ties that tend to be weak, she found. Conversely, timid birds form fewer associations that are long and stable. Both could be survival strategies. A risk-averse tit might prefer the greater protection from predators and disease that comes with hanging out with reliable friends. Less cautious birds, rubbing shoulders with many acquaintances, might get better information first—but risk exposure to every parasite and disease that spreads through the flock.
Aplin’s subsequent studies started revealing what social networks do. For example, they provide conduits for knowledge to flow from bird to bird. Seven times during the winter of 2010, Aplin visited Wytham Woods at night to put up new feeders while birds slept. In the following days, the birds most likely to find the new food were those that had the most connections to others that also had many social links. This connected-to-connectors factor is called Eigenvector centrality, but could be called the LinkedIn effect: It’s not just who you know, it’s who knows who you know.
And just like on LinkedIn, bird connections have consequences. Every one unit of connection that a bird has to a well-informed tit of the same species raises his or her chances of finding the new source of food by 22-fold, the team calculated.
Aplin realized that with this system, she could actually track the flow of information between birds. She captured Great Tits from eight different social groups and brought them into the lab, where they learned to solve a puzzle. Some learned to slide a blue door to the right to get mealworms. Others pushed a red door to the left. She then set up similar puzzle feeders with doors in the woods and released the trained birds. Puzzle-solving knowledge went viral. Birds were clearly learning from friends rather than figuring it out themselves: Those with red-door-sliding friends only slid the red door, and vice versa. She’d created avian memes.
The implications are heartening: Information can spread quickly, allowing birds to adapt to new circumstances far faster than genes alone permit. “In a world where the environment is rapidly changing because of factors like climate change and urbanization, the evolution of species won’t be enough to keep up,” Aplin says. With a little help from friends, they might close the gap.
I soon learn that I’m not as tough as a tit. By the end of the second round of banding my feet and hands are numb, and when Hillemann suggests a brisk walk through the woods to go check on her experiment, I gratefully follow her. The rest of the team keeps going. They will band nearly 100 birds in total that morning, and they’ll return next week. It’s a constant race to keep up, and it’s essential to the work they do. “It underpins the social network analysis—and all else,” says Firth.
That legacy of diligent data collection drew Firth to Sheldon’s group as a graduate student in 2012. Wytham was one of the only places in the world where a behavioral ecologist could test cause and effect in social networks with experiments in the wild. “I’m a sucker for an experiment,” he says. “It’s so much more elegant” than trying to draw conclusions based on observations. By disrupting the social system and watching the consequences, Firth calculated, he could see how the network really works. Such experiments also hint at what could happen to a population after an epidemic or catastrophe.
Working with Oxford’s physics department, Firth built the selective feeders, which Hillemann is now using. Squirrels annihilated the first version in one weekend; the sixth, squirrel-proofed with garden hoses and plastic containers, and also mud-proof, waterproof, woodpecker-proof, and protected by steel cages, was successfully deployed in 2013.
From the RFID-tagged feeder data, Firth first identified the social circles, which include various species. Then he interrupted them by creating new feeding groups. He rigged 12 feeders so that half opened only for birds with an odd-numbered chip, and the other half opened for evens. Odds could eat beside only odds, and evens with evens. It’s as if you checked Facebook one day and half your real friends were replaced by “friends” you didn’t know—and you had to have lunch with them.
By winter’s end, and more than 3 million records from 376 birds later, it was clear that tits were willing to make new friends for seeds. These Firth-created friendships carried over into other contexts: Even at ordinary feeders the birds stuck with these new friends. They also paid more attention to their new friends than to other birds. “Not only did they learn from the individuals that we’d forced them to hang around with, but they prioritized that information,” says Firth. He’d proved that the social network doesn’t just follow an internal logic; external forces such as access to food can reshape it.
Other social connections, though, resisted his manipulations. Firth tried to split apart couples by programming the feeders to open only for one bird of a mated pair. The heart, he found, was stronger than the stomach: Birds stuck by their mates, even though it was harder to get seeds. “They prioritize the pair bond rather than where to go for food,” he says. “It reminded me of sitting in a restaurant with my girlfriend’s friends, thinking, ‘I wouldn’t be with these people except that they’re with my partner.’ “
But even among mere friends, bonds can be surprisingly resilient. In a 2017 study, Firth and others analyzed 395,113 data points from 542 Great Tits, applying machine learning to process interrelationships, identify groups, and calculate the strength of relationships between individuals. Then Firth trapped six birds at a time, keeping them in a cage for several days. For some birds, half their flock had suddenly vanished.
Simulation experiments predict that if enough central nodes of an animal network disappear, the whole network collapses. To their surprise, what was left of the flock knit itself together again within a few days. The remaining birds responded by tightening their bonds, in some cases spending roughly 20 percent more time with the remaining flockmates, who in turn spent about 20 percent more time with them, increasing the overall network strength. Not only that, birds that were socially central maintained that position even if most of their friends were removed; they simply made new connections. “That’s something you never would’ve been able to predict without an experiment,” Firth says.
This self-repair mechanism is another hopeful discovery. It suggests birds faced with network-damaging catastrophes may be more resilient than we thought. A study published around the same time showed that people on Facebook do basically the same: When a mutual connection dies, friends and acquaintances reconnect, drawing the social network tighter. We usually assume that human social systems are far more complex than animal ones; this parallel, Firth says, suggests that simpler rules may govern both bird and human.
Wytham tits spend autumn and winter like this, roaming in gossipy little packs, surviving the lean months by joining forces. As spring approaches, groups begin prospecting for nest sites together. The birds eventually pair off. Winter banding and feeder experiments wind to an end. For the scientists, that only means more work. Predawn trips to check nest boxes and band any unmarked birds replace dawn banding parties. They monitor each of the 1,204 boxes for signs of nest building, the first egg, and clutch size, just as others have for all those decades. They band the chicks—and then, because as many as half die in the nest, they must later dig out the bands after the survivors fledge. “It’s a super-disgusting time,” says Firth cheerfully. “A soup of former birds,” chimes in McMahon.
Firth’s next experiment, underway this winter, involves a new feeder that opens only when two specific birds arrive simultaneously; they must work together in order to eat. It’s a way to probe whether social ties make it easier for birds to cooperate, or whether the opposite is true, whether joining forces forges enduring friendships.
If that sounds relatable—if, in fact, these Wytham studies remind you of friendships you’ve formed and lost, social groups you’ve joined or fled—there’s a reason.
Figuring out why wild animals have social circles, and how those networks function, is fascinating in its own right. In addition, it will cast new light on why some populations thrive and others fade away, and it will guide conservationists as they help species cope with threats to survival.
But this research could answer other questions as well. Because we, too, are social animals, the insights found may help us better comprehend our own behavior.
Despite their immense reach, companies like Facebook are limited in what they learn because they track just one species. As behavioral ecologists apply the ideas and methods of social network analysis across the animal kingdom, describing, testing, and disrupting the friendships of many creatures, they expect to find common patterns and principles that organize the worlds of Blue Tits, blue whales, and everything in between.
People tend to say that animals are just like us, but it may be that we are just like them. The universal rules of animal friendship may deepen our understanding of how our connections form and erode and why we cluster together as we do. Perhaps they will reveal why friendships feel so essential and are literally life-sustaining: Psychologists know that people with friends live longer. For all social creatures, including tits and humans alike, connections are as fundamental as food, water, and shelter. These little birds may show us why.
This story originally ran in the Winter 2018 issue as “The Social Network.” To receive our print magazine, become a member by making a donation today.