Invading Snowy Owls Are Leaving a Trail of Lemming Corpses in Their Wake

Every winter, North American bird-watchers look forward to catching a rare glimpse of the mysterious snowy owl. The Arctic animals push south in the frigid winters, and a few hundred end up breaching the 49th parallel if they're feeling particularly adventurous.

But this year is different. Snowy owls are barreling across the border by the thousands, creating the largest “irruption”—the ecological term for a sudden population increase in five decades. To give you a sense of the scale of this migratory invasion, let's take data from a single state: Ohio. In 2012, the number of snowy owl sightings capped at 15, which exceeded the annual average. Ohioan birdwatchers rejoiced the banner year.

Flash forward to the never-ending winter of 2013-14, and over 158 owls have been spotted in the state so far. Indeed, some snowy owls have been seen holidaying as far south as Florida, cementing the state's reputation as a choice winter vacation spot for all species. “Something huge is going on," said Dave Brinker, a biologist with the Maryland Department of Natural Resources. “We won't see something like this for a long time—probably for the rest of our lifetimes."

The owl explosion was catalyzed by the awesome year their main food source—lemmings—have enjoyed in the Arctic tundra. But there's a little more to the situation than a lucky year. Until the mid-1990s, the lemming population went through an interesting boom-and-bust pattern lasting approximately four years, creatively called the “lemming cycle.”

The cycle greatly affected the population of the small rodent's predators, snowy owls being premiere among them. When lemming populations spike, owl populations naturally spike with them, due to the excessive nom noms. But the huge increase in mature owls leads to a corresponding decrease in the lemming population, which in turn precipitates a bust for both species. Voila: the whole cycle starts all over again.

But during the last two decades, climate change has disrupted the entire process, and ecologists are still trying to shift through the bizarre and random population patterns that have replaced the regular lemming cycle.

“Alpine and arctic lemming populations appear to be highly sensitive to climate change, and when faced with warmer and shorter winters, their well-known high-amplitude population cycles may collapse,” a 2012 study from the Royal Society read. “Being keystone species in tundra ecosystems, changed lemming dynamics may convey significant knock-on effects on trophically linked species.” The problem is especially exacerbated by reduced snow and ice at the Northern polar cap. Lemmings need that snow to hide from their predators and produce healthy new generations.

The upshot of all this is that on a macro scale, lemming populations are decreasing, but for whatever reason, last summer there was a giant spike in their reproductive success—the first in a decade. Perhaps due to a particularly snowy winter, lemmings were able to slut it up like crazy, and the delicious new buffet did not go unnoticed by the owls. Indeed, a telltale sign of the bountiful rodent harvest was observed last July by biologist Jean-Francois Therrien, who found an owl's nest lined with 70 dead lemmings.

“I’ve encountered some lemmings around nests in the past, maybe one to two, or three,” Therrien said. But 70 dead animals is completely unprecedented, and it was not an isolated case. A nearby nest was decked out with 64 fallen lemming soldiers, which means that the chicks that came of age last autumn were the most well-nourished snowy owl generations in decades.

There is a huge misconception floating around that this massive snowy owl irruption is due to food shortages in the Arctic, forcing the birds farther south in search of sustenance. The opposite is true: the reason we're seeing more owls is simply because there are a lot more owls this year. Indeed, many of the biologists who have been taking advantage of the boom have noted that the owls they've tagged are looking pretty “high on the hog,” which is a nice way of saying these birds are total fatties.

But one good year for the snowy owls is a drop in the bucket compared to the enormous challenges the species faces in the coming decades. The Arctic is warming faster than the globe as a whole, and the good times are going to be few and far between. Sorry owls: this may the last time you have a hoot for some time.