For want of snow, the tawny owl of Finland has become more brown in the past half-century, according to new research published February 22 in Nature Communications. Finnish researchers scored tawny owls (Strix aluco)—a raptor common to all of Europe—on the color of their plumage, specifically how brown (dark) or gray (pale) their feathers were. (Scientific American is part of the Nature Publishing Group.)

Since the owls do not seem to show a sexual preference for either darker or lighter feathers, the main driving force for any change in plumage would be natural selection, in this case, most likely, whether a dark or a light coat helped in catching more voles, the raptor's primary food. Over the course of the study led by bird ecologist Patrik Karell of the University of Helsinki—from 1981 to 2008—the brown owls started to increase in numbers.

More specifically, more brown owls survived Finnish winters. The reason? Unknown for sure, but Finnish winters have become milder in recent decades thanks to climate change—with much less snow. Less snow means less chance of brown owls standing out against a white landscape as they stalk their prey—or falling prey to the dread Bubo bubo (the eagle owl). Or it may be that the brown coloration is also tied to the owl's physiology, like the strength of its immune system or how much food it needs to eat.

In fact, since at least 1961, the proportion of brown tawny owls caught by ornithologists throughout Finland has increased, adding more circumstantial evidence in support of the finding.

At the same time, there is no direct evidence yet to support any of those possible explanations for why brown owls would do any better than gray owls, even in milder winters, as the researchers admit. In fact, browner and grayer owls have roughly the same level of survival through winter these days (an improvement for the browns and stasis for the grays).

Nevertheless, the Finnish tawny owl may join a relatively short list of animals that have been shown to have evolved in response to human-induced changes to the environment. There is the famous peppered moth of the U.K., which flipped from light-colored to dark-colored as a result of the discoloration of its habitat by coal soot in the 19th century and then back again towards the end of the 20th century as those conditions cleared up. More recently, the bottom-dwelling tomcod in the Hudson River have rapidly evolved a resistance to polychlorinated biphenyls—better known as PCBs and once used in electrical equipment—that were dumped in the river by General Electric in the middle decades of the 20th century. And a wide range of animals and plants are changing behaviors or range—and potentially ultimately their very genetics—in response to the changing climate as a result of greenhouse gas emissions from fossil fuel burning.

But the more biologists look, the more often the human scientists are finding that our own species is directing evolution for other species, albeit unwittingly.

Image: Courtesy of Dick Forsman