Evolution is one of nature’s greatest shows. From humble beginnings, it gave rise to fish, reptiles, amphibians, birds and mammals; it took the ancestors of apes and sculpted them into humans. These grand spectacles play out across millions of years, dazzling us with their before-and-after shots. But if we look closely, we can sometimes glimpse evolution unfolding in real time. And often we are surprised to find ourselves directing the show.

A new study finds that great tits, a type of songbird, in the U.K. have evolved longer beaks over the past couple of decades, possibly because of a British pastime that has become massively popular since the 19th century—bird feeding. The findings, reported Thursday in Science, represent a possible empirical example of an organism evolving directly in response to human activities.

The great tit, a black-and-yellow bird with a call like a squeaky seesaw, is common across Europe and is a frequent visitor to bird feeders. Biologists have long known U.K. great tits had longer beaks than those in continental Europe, but the reason for this difference was unclear. Now a group of Dutch and British researchers tracked the variation down to the birds’ genes and found natural selection at work.

They first embarked on an ambitious investigation into the genomes—the complete sets of genes—of 2,322 great tits from across the U.K. and the Netherlands. They flagged spots in the genomes between the two populations that were most different, which turned out to be the same spots that were the least different among the genomes of individual U.K. birds. “Favored” genes had nudged their way into these areas and, with time, pushed out most other genes that might have been found there—a clear indication natural selection was at work, according to the researchers. They then labeled around 30 of the genes that seemed to oust all the others and scanned the scientific literature to figure out what role these genes played in other organisms. It turns out many of them are involved in human skull development and bill length in Darwin’s finches—Galápagos Islands birds that served as the famed biologist’s most famous subjects.

The scientists suspected these genes could be linked to beak length, so they measured this anatomical feature in U.K. great tits. They found birds with that handful of favored genes, as expected, tended to have longer beaks than their continental counterparts in the Netherlands. The researchers further backed their findings using historical data. Ecologists at the University of Oxford, which co-authored the study, had been measuring beak lengths for 26 years, or around 13 generations for the songbird. “Even just in the last 30 years we’ve seen the beak lengths getting longer in the U.K. population,” says senior author Jon Slate, an evolutionary geneticist at the University of Sheffield. And he thinks the evolutionary time frame is even longer than that. The researchers then scoured museum collections and looked at specimens of the birds collected since the 1920s to confirm their findings. Something about the U.K. environment was giving longer beaks an edge.

“It was a fishing expedition,” says David Reznick, an evolutionary biologist at the University of California, Riverside, who did not take part in the study. “They have done a good job of identifying the groups of genes that are associated with variation in [beak length].” The signature of natural selection in their data, he adds, is quite strong.

Leif Andersson, a geneticist at Uppsala University in Sweden, also not part of the work but who has conducted research on the genes that drove natural selection in Darwin’s finches, was impressed by the amount of data the team generated. “We have only used hundreds of birds and identified this association [of evolving bill length],” he says. “They have used thousands.”

Human Touch

“What makes a bird with a long beak better than a bird with a short beak?” Slate asks. There are documented instances—such as studies on Darwin’s finches—that show evolution of beak length results from differences in food availability. Peter and Rosemary Grant, a famous pair of evolutionary biologists and emeritus professors at Princeton University, for example, spent 40 years observing Darwin’s finches in the Galápagos. They found that finch beak size evolved based on competition for dwindling food sources in extreme climatic conditions such as drought. But the U.K. great tits are not known to have different diets than those in the rest of Europe, according to the researchers. After consulting with some ecologists, the team decided to test whether this natural selection had something to do with bird feeders. After all, according to a 2015 study, around half of U.K. homes feed birds in their gardens. And the U.K. spends twice as much on birdseed than the whole of continental Europe, according to a study cited by the researchers.

Ecologists in the new study gauged the “fitness” of great tits by counting the number of chicks the birds reared to maturity in artificial nests. They found that in the U.K. the great tits with the genes for shorter beaks had fewer surviving chicks per year than those with longer ones. According to Slate, although the difference is small this probably means the latter were better able to bring seeds back to their young. The researchers also tagged the birds to count the amount of times they visited the feeders, finding longer-beaked ones showed up at the feeders more often than their more stubby-beaked cousins. “So they seem to be better at exploiting that resource,” says Slate, a bird feeder himself. “If that part of the story is true, it tells us that humans have actually been the selective agents, which [have] caused this evolutionary change in great tits.”

But that’s just an “if.” Slate cautions this is just a hypothesis and needs much more data to be confirmed. It is also unclear what it is about bird feeders that give longer-beaked great tits an advantage. Reznick agrees that more data is needed, but he is optimistic this idea is testable. “There may be other explanations for what they’re saying but they’ve done a good job of making a case for [the bird feeders] as one possible explanation—and it really is a testable hypothesis,” he says.

Yet the idea humans may be influencing evolution is not new. One well-known example involves moths that evolved a darker coloration as camouflage in the soot-covered backdrop of the industrial revolution. “A human influence on beak-size evolution is not new, we have seen signs of this in finches on an inhabited island (Santa Cruz) of the Galápagos, but an association with bird feeders is new as far as I know,” Princeton’s Grant wrote in an e-mail. Speaking of “the human touch on evolution, I would point to current global warming, partly caused by human activities, and the likelihood that animal and plant populations are undergoing evolutionary change by natural selection in response,” he added.

Slate hopes to pursue more studies that may help answer the mysterious influence of the bird feeders but he does note the difficulty of drawing major conclusions because of the lack of literature on how widespread the practice is. He also hopes to extend the range of study to the rest of Europe to understand how one species has adapted so well to very different environments.

All the scientists agree long-term studies like this are valuable in seeing—in real time—a process that continuously alters history and the ecosystem. “In the public perception, when we say evolution they expect that you’re going to go outside and see a different world,” Reznick says. But even these beak lengths only differed by a fraction of a millimeter. He adds: “The noticeable things are sort of the large-scale version of the smaller things that are going on all the time, and that evolution is probably happening in organisms all around us all the time—but you need to know how to look for it.”