If you’ve spent much time observing birds—you clicked on this nerdy story, so that feels like a safe bet—you probably have noticed them wiping their bills on a tree branch or fencepost, or whatever else they’re perched on. And you might have wondered: What’s going on there?
A few things, it turns out. Bill-wiping is not the hottest topic in ornithology, but curiosity has drawn the occasional researcher to the behavior over the years. Although they haven’t arrived at a definite, universal explanation, we can summarize their reports on the role of bill-wiping this way: It definitely acts like a napkin, probably as a file, and maybe even as a cologne spritzer.
The first scientific paper to focus on the behavior, it seems, was a 1970 review by ornithologist George A. Clark, Jr. “Bill-wiping typically involves rapid withdrawal of the side of the beak from base to tip closely adjacent to a foreign surface such as a branch or the ground,” he wrote. “I have seen passerines wipe on rope clothesline, fence wire, the edge of a metal birdbath, and the rim of a metal incinerator.” While it often involves just a few swipes here and there, Clark cited one report of a finch wiping its beak 90 times in a few minutes. His research turned up more than 90 species known to engage in the activity, and he surmised that all birds do it, with the possible exceptions of hummingbirds and waterfowl. (Counterpoint: I dunno, this hummingbird sure seems to be wiping its bill.)
The napkin principle—that birds wipe their bills primarily to clean them—is generally agreed upon, based on logic and observation. “As widely noted, birds frequently bill-wipe after eating messy foods such as suet, fruits, or juicy insects,” Clark wrote. It just makes sense; you know how it is when you’re eating juicy insects. To make sure, though, researchers in the U.K. did an experiment and confirmed in a 1992 study that European Starlings wiped their bills more often after eating sticky food than dry food.
A larger goal of that study, however, was to test the file hypothesis—the idea that birds wipe their bills in part to shape them. It was prompted by a student who saw a starling wipe its bill and asked lead researcher Innes Cuthill the reason for the behavior. Cuthill didn’t know the answer, so he searched a library and found that no one had studied the question in a lab. “I reasoned it was probably like cats sharpening their claws, or rodents chewing to keep their teeth down, so that led to the experiment,” Cuthill, an ecologist at the University of Bristol, said in an email.
Like fingernails or hair, the outer portion of a bird’s beak is made of the protein keratin and grows nonstop. Foraging and feeding wears this outer layer, giving the bill its shape. Starlings and other species, Cuthill and colleagues noted, shift their diets at different times of year, eating mainly bugs and worms during breeding, but switching to lots of fruits and seeds in fall and winter. They wondered if bill-wiping might help birds hone their beaks into shapes that work best for grabbing whatever type of food they’re focused on.
The answer, their findings suggest, is yes. Wiping had a significant impact on bill length and shape, they found. Birds that were assigned to smooth perches wiped their beaks more often than those with rough ones, apparently compensating for the lack of abrasion. And birds with rough perches to rub against turned out to be faster at picking up food than those with smooth ones. “This lends support to the idea that wiping frequency may be strategically adjusted to tune bill shape to current diet,” the team wrote.
Along with cleaning and honing, scientists have noticed that bill-wiping seems to happen a lot in social interactions between birds. Back in 1970, Clark’s paper characterized it in those situations as probably a “displacement activity.” That’s a term for behavior like fidgeting or head-scratching that arises unconsciously when you’re frustrated or conflicted, and it’s a concept that’s lost currency among scientists. “We don’t really accept those kinds of explanations for animal behavior anymore,” says Danielle Whittaker, an evolutionary biologist at Michigan State University. “We look to see if there’s anything functional.”
Which brings us to the cologne hypothesis. Whittaker is fascinated by smells and how they play into animal reproduction. Her research has shown that preen oil, which birds produce through a gland and use to maintain and waterproof their feathers, contains odors and chemical signals that play a role in mate choice. So when she became aware of bill-wiping, Whittaker wondered if birds might be slathering preen oil on nearby surfaces to release those smells and lure a mate.
To find out, she conducted what she calls “a fun little experiment” with Dark-eyed Juncos in Grand Teton National Park. It involved placing a caged junco—sometimes a male, sometimes a female—in the middle of a wild junco’s range, paired accordingly with a recording of a female’s come-hither trill or a male’s courtship or territorial songs. Whittaker and team videotaped the wild bird so they could count its bill-wiping, and, in 2014, reported evidence that the behavior plays a part in junco courtship. “I didn’t see it very often in response to another male,” she says, “but I did see it very reliably in response to a female.”
These findings suggest that, routine and subtle as it seems, bill-wiping may have a meaningful role in the most essential aspects of avian life. Like pretty much everything about birds, its fascination deepens the more you learn.