Human hunters may be making birds smarter by inadvertently shooting those with smaller brains. That’s the conclusion of a new study, which finds that hunting may be exerting a powerful evolutionary force on bird populations in Denmark, and likely wherever birds are hunted. But the work also raises a red flag for some researchers who question whether the evolution of brain size can ever be tied to a single factor.

The new work “broadens an emerging view that smarts really do matter in the natural, and increasingly human-dominated, world,” says John Marzluff, a wildlife biologist and expert on crow cognition at the University of Washington in Seattle who was not involved with the work.

Hunting and fishing are known to affect many animal populations. For instance, the pike-perch in the Finnish Archipelago Sea has become smaller over time thanks to fishing, which typically removes the largest individuals from a population. This pressure also causes fish to reach sexual maturity earlier. On land, natural predators like arctic foxes and polar bears can also drive their prey species to become smarter because predators are most likely to catch those with smaller brains. For instance, a recent study showed that common eiders (maritime ducks) that raise the most chicks also have the largest heads and are better at forming protective neighborhood alliances than ducks with smaller heads—and presumably, brains.

Does the same hold true for birds that dodge human hunters? To find out, Anders Pape Møller, an evolutionary biologist at the University of Paris-Sud, assessed the brain sizes of 3781 birds from 197 species brought to taxidermists in Denmark between 1960 and 2015. The birds included pheasants, partridges, wood grouse, magpies, and hooded crows. Danish law requires taxidermists to record the date and cause of death of every specimen they handle. Møller’s co-author, Johannes Erritzøe, a taxidermist and ornithologist at the House of Bird Research in Christiansfeld, Denmark, autopsied each bird, noted its mass, and weighed its extracted brain. The scientists also assessed the birds’ body condition and age at death.

They found that 299, or 7.9%, of the 3781 birds were shot. Birds with smaller brains relative to their body size were shot more often, as were larger individuals (which offer a bigger target), and males (perhaps because of their brighter colors). But if a bird had a large brain relative to its body size, the probability that it would be shot decreased nearly 30-fold, the scientists report today in Biology Letters . This held true, regardless of the birds’ health, body mass, sex, and species. Hunters, they conclude, are unwittingly turning their prey into large-brained birds by eliminating those with pea-sized brains from the population.

The scientists also compared the birds' other internal organs—heart, liver, lungs—and found that only the brain was smaller in the hunted birds. “It means that hunting has a very peculiar and specific effect on the brain and not the other bodily functions of these animals,” Møller says.

Hunters aren’t specifically targeting the smaller-brained birds, he adds. Such birds simply aren’t savvy about hunters, apparently lacking the smarts to realize that people with guns are dangerous. “They take longer to fly when approached by someone with a gun, whereas larger-brained birds enjoy the benefit of being wary.” Møller and his team couldn’t track changes in brain size over time, because hunting regulations in many of the study areas have shifted; there are some areas where it was once allowed, but it’s now banned. That could let birds with smaller brains gradually make up a larger part of the population, Møller predicts.

“The study is intriguing, but I will remain a bit skeptical because it is based on a comparative long-term data set and not an experiment,” says Jesper Madsen, a population ecologist at Aarhaus University in Rønde, Denmark, who was not involved in the study. “To conclude that hunting selects for larger brains requires more than a correlational study.”

Such an experiment is already underway—albeit inadvertently, Møller says. In the last 5 years, the hunting of snipe and curlew has been banned permanently in Europe. Scientists could compare specimens from the earlier hunting period with those collected after the ban to see whether these birds are evolving smaller brains, Møller says. “That’s a predictable consequence of stopping hunting.”

Still, Madsen isn’t alone in his skepticism. “My heart drops every time I see another study like this showing a correlation between some factor and brain size,” says Susan Healy, an evolutionary biologist at the University of St. Andrews in the United Kingdom. In 2007, she and Candy Rowe, a behavioral ecologist at Newcastle University in the United Kingdom, assessed more than 50 studies that revealed a correlation between brain size and behavioral traits such as migration, deception, and female promiscuity. They concluded that this type of research did little to advance an understanding of either brain evolution or function.

Healy’s and Madsen’s concerns are valid, Marzluff says. But, he adds, the study is valuable because of the questions it raises. “For example, did smarts pay off more for some species than for others? Were similar trends seen in social versus solitary species? That’s what correlational studies do: They generate questions.”

Indeed, the authors raise several at the end of their study. To wit: If hunters are indeed making birds smarter, what will this do long-term to bird populations and to the sport of hunting itself? Will these birds be increasingly harder to catch, for example? And how would this affect wild predators that live on these species? Møller predicts they’ll have a harder time. As for the answer, stay tuned.