With their powdery white feathers and haunting yellow eyes, snowy owls are one of the most iconic animals of the Arctic. They’re also one of the only ones that makes regular visits into the non-Arctic, with jaw-dropping owl blizzards making regular appearances in southern Canada and the northern United States during their annual winter migration.

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This year we’ve been witnessing a “mega-irruption”—an irregular, dramatic migration—with owls sighted in places as far-flung as New York state and Odessa, Texas, and even atop the U.S. Department of Agriculture building in Washington, DC. Needless to say, bird enthusiasts are having a field day.

Yet this seeming abundance of snowies masks the unfortunate fact that these charismatic birds are in more danger than ever before. Exactly what threats they’re facing has been tough to suss out, because snowy owls don’t have easy-to-trace regular migrations; they’re “highly nomadic at all points in their life cycle,” says Scott Weidensaul, a Pennsylvania naturalist and owl researcher who runs a program to track these birds on their far-flung travels.

For scientists, where snowy owls go and what they do throughout the year is still largely mysterious—which is becoming a problem as climate threats to the birds mount.

In December 2017, the International Union for Conservation of Nature changed the snowy owl’s status to “vulnerable” on its updated Red List of endangered species in light of new research. That designation will allow researchers to monitor the species with more scrutiny and better argue for their conservation, says wildlife biologist Denver Holt, founder of the Owl Research Institute. “The snowy owls are an indicator, in my mind, of the health of the Arctic environment,” he says. “They’re also clearly the avian icon of Arctic conservation.”

Until recently, researchers estimated that there were 300,000 owls (including 140,000 in North America) in the wild, a number extrapolated from an early-2000s population sample from one portion of Arctic tundra taken during peak season. In 2013, Bryn Athyn College biologist Eugene Potapov and Arctic expert Richard Sale challenged that estimate, saying it didn’t reflect snow owl cycles and their nomadic lifestyle. In their book The Snowy Owl, they took a different approach, looking at owls during breading seasons across the tundra subzones to find that their population was more like 30,000—though the authors caution that even that is simply “a guesstimate.”

In his annual research trips, Potapov has witnessed a changing Arctic, with transformed snow conditions and melted sea ice. Based on this rapid environmental change, he and others believe the snowy owl population may be even lower. In its 2016 annual report, bird research and conservation organization Partners In Flight noted that the snowy owl population is “believed to be rapidly declining” while acknowledging that “populations are difficult to estimate.”

The snowy owl’s irregular movements are tied to a semi-regular natural process: the lemming population cycle. Lemmings may be best known for the urban myth of jumping off cliffs en masse (which dates back to a 1950s Disney “documentary” that involved manually driving lemmings off of a cliff). In reality, they a key food source for the snowy owl. But there’s a lot of boom and bust in the lemming population, meaning that means every few years—around four years in many areas across the Arctic—an extra-cold year with fluffy insulating snow creates the perfect conditions for these rodents to have lots and lots of delicious babies.

A high lemming year is a feast for carnivores like the Arctic fox, the Arctic wolf, and, of course, the snowy owl. The raptors, who like every other Arctic species live in extreme conditions, rely on the wealth of prey provided by a lemming boom to have a good breeding season. After they breed, snowy owls head south in great numbers for the winter. This year’s owl boom is an echo of the 2013 snowy “mega-irruption,” when an estimated 8,000 birds headed south to the United States, reaching as far as Florida and Bermuda.

Previously, scientists believed snowy owls irrupted because they were starving in the Arctic, having exhausted their lemming supply. However, it turns out that the snowy owls who come south actually tend to be relatively healthy and well-fed. Weidensaul says that irruptions may actually signal a boom year for the birds, when so many have bred that they can’t all stay in the Arctic, on sea ice or in the tundra, throughout the scarce winter.

During an irruption, younger owls strike out on their own in search of food and space. That quest kills many: the low-swooping birds get hit by vehicles, attacked by other raptors such as eagles, or poisoned by eating prey that has been exposed to rodenticides. Yet their fates, as well as their non-Arctic activities, are still poorly understood.

Weidensaul aims to change that. He is also the cofounder of Project SNOWstorm, which tracks the “winter movement ecology” of individual snowy owls. For the past five years, the project has been following around 65 individual owls that have been tagged using tiny solar-powered trackers attached to the birds like backpacks.

The trackers offer researchers an unprecedented amount of data on where the birds are, how they interact when they’re near each other, and what kinds of habitat they prefer. When the birds head out of cell range, the trackers store data and transmit it when they’re back in range, which means that even when they’re back up in the Arctic, chances are researchers will be able to collect their data when they head south again.

The information from these trackers has helped to confirm that many snowy owls who come south are in good health, partly by enabling dead birds to be found and analyzed. It’s also revealed that the snowies have wildly different habits: , while some birds cover thousands of miles over their wintering season, flying from place to place, others don’t move around very much at all. Those include Badger and Arlington, two owls that have stayed close to where they were tagged in Wisconsin during the 2017-2018 winter.

The data Badger, Arlington and their fellows collect helps conservationists make decisions that help snowies survive their changing world. A big part of that is an interruption to their stable relationship with lemmings. “The Arctic has changed,” Potapov says. “So you’ll see more irruptions and less breeding.”

In the meantime, know that the out-of-place owls you enjoy spotting outside the Arctic come with an important backstory. Snowy owls have be referred to as “possibly the world's sexiest bird”—but for scientists, they are also one of the world’s most mysterious.