In Bird Feeder Battles, Social Species are Featherweights
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Anyone with a bird feeder knows that winter can be a season of high drama. With wild food sources hard to find, offerings of seed and suet can draw a crowd–and lots of tussling. Watch closely, and you’ll see winners and losers. Some birds stand their ground, and others flee at the first sign of conflict.
Bigger birds tend to be victorious in these skirmishes, but a new study published in The Proceedings of the Royal Society considered a subtler factor that affects feeder hierarchy: social life. It turns out, the most social birds–the ones that tend to show up in a group–are the least likely to win a face-off against an outside challenger of comparable size.
That might seem counterintuitive to an avid feeder-watcher, who knows these birds get plenty of sparring practice among their peers. The finding surprised the study’s authors, too. “My assumption was that the more social species would be more powerful for their body size,” says Roslyn Dakin, a behavioral ecologist at Carleton University in Ontario, Canada, and senior author of the study. “But what we found was quite the opposite.”
Understanding the pecking order helps researchers see the bigger picture of how species interact in an ecosystem and can shed light on the evolution of different traits and behaviors. Previous studies of feeder battles established that body size matters most. Bigger, heavier birds drive off smaller species and “win” the most squabbles. Longer bills also help. But Carleton Ph.D. student Ilias Berberi, first author of the new paper, wanted to look past physical traits. “There’s a lot more to animal biology than just an individual’s measurements,” he says. “The behavior of animals has such an influence on how well they can survive.”
The problem was how to study it. Research on intangibles like social dynamics is inherently challenging, and it became harder still when the arrival of COVID-19 scuttled Berberi’s original plan to study competition between hummingbirds in the laboratory. While he was looking for a remote-friendly project, Dakin, his adviser, told him about Project FeederWatch, a long-running community science effort coordinated by the Cornell Lab of Ornithology. Volunteers across the United States and Canada report not only the birds they see at their feeders but also the interactions between them. Every showdown between junco and nuthatch or House Finch and Tufted Titmouse gets recorded, along with weather data and other details like the arrival of predator birds.
“It’s incredibly rich,” Berberi says, of the FeederWatch dataset. “I was like ding-ding-ding, this is the one.”
Berberi, Dakin, and their collaborator at Cornell, Eliot Miller, pulled the data from four recent winters of observations–Project FeederWatch runs each year between November and April–and narrowed their focus to 68 common species. That gave them a dataset of over 55,000 “displacement interactions,” or instances when a single bird drove off another individual of a different species.
“Those evictions aren’t random,” Dakin says, “They’re a reflection of a dominance hierarchy.”
Individual birds from more social species, like goldfinches, Common Redpolls, and Black-capped Chickadees, had little success in one-on-one conflicts with birds of other species. Dakin calls these kinds of birds “groupy and wimpy.” On the other end of the spectrum, some more solitary birds were far more dominant than their size alone would predict. Downy Woodpeckers and Carolina Wrens, for example, are relatively small but nonetheless frequently manage to drive off competitors.
While the findings showed a clear pattern of dominance by solitary birds in size-matched fights, the results also offer hope for the “wimpy” birds: Having companions nearby can give social birds an edge, even when their friends stay on the sidelines. “Sometimes the underdog can get a boost in competitiveness based on who’s around them,” Dakin says.
Group life does have its pros. “Species can gain so many benefits from being social,” says Berberi. Communal living can make it easier for birds to find mates, defend territory, and fend off predators. But the study’s findings suggest these advantages may come at the cost of individual competitiveness, at least at the bird feeder.
“It’s kind of mysterious,” says Gavin Leighton, an evolutionary biologist at SUNY Buffalo State who was not involved with the study. “It’s hard to fathom why that would be a trade-off.” Leighton published a paper last year that also analyzed FeederWatch data and included a related finding: Species that fight more among themselves–as social species tend to do–are less likely to dominate other kinds of birds. Even knowing that, Leighton was surprised the new analysis revealed such a clear pattern.
For now, the authors can only speculate about what’s behind the apparent evolutionary compromise between sociality and individual competitiveness. Their paper offers several possibilities: Perhaps social species simply must prioritize competing with each other over fighting with outsiders. Or maybe group foraging is so effective that feuding with other kinds of birds is unnecessary.
Another mystery: Why doesn’t the strength-in-numbers effect hold true for every species? The Pine Siskin, for example, “is a very social species that will always lose by itself,” Berberi says. But just having more siskins nearby can increase a individual’s odds of victory in a one-on-one conflict with another species, even fierce loners like woodpeckers. Other birds, like the Northern Cardinal, tend to fare worse in fights with more of their kin around. It’s not clear what’s behind the variation, although the more social the species, the more they seem to benefit from having company.
The authors note the sociality of birds is a lot more complex than how many appear together at a feeder. In some species, individuals come and go, while in others they form long-term bonds. And many birds form mixed-species flocks for at least some of the year, especially in winter.
“The next step is to explore the structure and the dynamics of social interactions,” Berberi says. “It’s really exciting to see these future ideas grow.”
Berberi and Dakin still plan to get back to their in-lab hummingbird studies, but they say they’ll keep working with Project FeederWatch data as well. “Through the power of lots and lots of people, we can answer questions that no one ecologist would be able to answer on their own,” Dakin says. “It would take an ecologist like a hundred years to collect that many observations.”
Besides the immense dataset it makes available, Dakin says FeederWatch has inspired her work in another way, too. “I’ve got to watch my own feeder,” she says. “This could be a source of new hypotheses.”