Burnham has noticed the change while doing fieldwork in the high Arctic, too. “A warm day meant 40 degrees Fahrenheit in the early ’90s. Now a warm day means it’s in the low 60s,” he told me. This extra warmth has extended the breeding window for peregrines, letting them survive in Greenland later in the year.

That by itself is crucial. As its name suggests, a peregrine—“one from abroad”—is a migratory bird. Every spring, peregrines fly from their winter nests in the tropics to summer nests in the north. Their range seems to expand on both sides at once. “The further north a bird gets, the further south it goes,” Burnham says.

This means that the peregrines north of Thule have incredible, almost incomprehensibly lengthy migrations. Burnham estimated that a peregrine from northern Greenland migrates more than 8,000 miles south, to Costa Rica, Panama, or Venezuela. Meanwhile, a peregrine falcon who summers in Washington, D.C., might only winter in the Caribbean or the Texas Hill Country.

Greenland-bound peregrines also wait until later in the year to set out, delaying their northerly flight until late May or early June. By that time, the Texas peregrines haven’t only made it to Washington; they’ve also laid their eggs and hatched their chicks.

If peregrines are the nomadic generalists of avian world, then gyrfalcons are the specialists. With a warm feather layer, gyrfalcons have evolved over the last hundred thousand years to thrive in the high Arctic. Some Greenlandic gyrfalcons may spend the entire year north of the Arctic Circle. They may alight on icebergs, riding them into the open ocean for weeks at a time, leaving them only to stretch their wings or hunt for seabirds.

Gyrfalcons don’t build their own nests. Instead, they appropriate other birds’ nests in the early spring—moving into a nest built by a raven, for instance, in the previous year. Gyrfalcons will then return to that borrowed nest year after year. Eventually, the original sticks and mud that constituted the nest are entirely obscured by centuries of guano.

“You see them three feet wide and three feet thick, with no sticks. The guano is just like concrete,” says Burnham. Recently, researchers cored gyrfalcon nests, reading the layers like tree rings. Some of the nests were more than 2,500 years old. Gyrfalcons are raising their young today in nests that are older than the Roman Empire.

Except that the peregrines are starting to evict them. Like gyrs, they also move into nests built by other birds, and, as they explore the north, they are finding gyrfalcon homes quite comfortable indeed. And unlike gyrfalcons—large but gentle creatures who will often just fly away if challenged—peregrines are aggressive, defending not only their new nest but also the hunting ground around it.

“It’s an F-16 against a Cessna. In one year, we found a gyrfalcon chick that had just learned to fly, on the ground with a broken wing,” says Burnham. “It was probably a result of a peregrine striking the gyrfalcon nest.”

Global warming may be helping peregrines move north right now, but not all of its changes have been positive for the birds. During the summer, it now rains regularly in Thule—the only precipitation used to be dry, frigid snowflakes—and, across Greenland, both snow and rain seem to be clumping into large storm systems. Instead of seeing three months of rain fall over the course of three non-consecutive weeks, an entire season’s worth of rain will now fall in three hours.