NUUK, Greenland—Far up the coast of this ice-dominated island—north of the Arctic Circle; north of the glacier that spawned the Titanic-sinking iceberg; and north of the northernmost American military base—two birds of prey are locked in a vicious battle for food and territory.

Kurt Burnham has spent the past decade watching the fight take shape. He studies falcons at the High Arctic Institute, in Orion, Illinois, and he has traveled to Greenland most of the summers of his life.

For many of those trips, he helped survey peregrine falcons that use western Greenland as a summer nesting ground. But about a decade ago, he began tracking something new. As climate change tempered the Arctic’s frigid summers, peregrines were expanding their range north—farther north, he found, than there were ever records of them traveling before. Peregrine pairs began returning, summer after summer, to nest on the island’s northernmost cliffs.

They were not alone there. Another bird of prey, the gyrfalcon, has nested on the same cliffs for millennia. Though it is specially adapted for high-Arctic life, and larger than the peregrine, this gentler and more conflict-averse bird is not prepared to compete with peregrines. Now, the peregrines regularly attack and overwhelm the gyrs.

The fight previews battles to come on a fast-warming planet. Global warming can sometimes sound like a passive phenomenon—as if wild animals just wake up one day to discover that the air is hotter and all their food is gone. But Greenland’s peregrine-gyr war suggests that the upheavals to come will be bloody ones. Climate change is not only triggering a kind of slow ecological impoverishment, but also bringing very real, very fast invasions.

Burnham is not optimistic about the contest between peregrines and gyrfalcons. “Over the next 10 to 15 years,” he says, “you’ll probably see one of them go extinct in the area.”

* * *

There are few greater environmental success stories than that of the peregrine falcon. Once widespread across North America, peregrine populations were devastated by the widespread introduction of the pesticide DDT in the 1950s. The chemical dispersed through the ecosystem and built up in the bodies of apex predators like peregrine falcons and bald eagles. It weakened female peregrines’ ability to create strong egg shells: When they were able to lay eggs at all, they sometimes crushed them under their own weight.

Rachel Carson’s 1962 book, Silent Spring, brought mass public attention to the crisis and helped spark the modern environmental movement. The U.S. government banned the chemical in 1972.

The quest to save peregrines began around the same time—and Burnham basically grew up inside it. In 1970, Tom Cade, a professor at Cornell University, founded the Peregrine Fund specifically to help conserve the endangered birds. Cade recruited one of his students, William Burnham—Kurt’s father—to accompany him on a trip to Greenland.

Burnham stuck around, as did his soon-to-be wife, Pat Burnham. In May 1975, the month that Kurt was born, the first falcons were hatching from the Peregrine Fund’s catch-and-breed program. William Burnham ultimately led the Peregrine Fund and the World Center for Birds of Prey for more than two decades.

While the Peregrine Fund worked on catch-and-breed programs in North America, the Burnhams’ trips to Greenland were limited to observation. Kurt or his father returned to western Greenland every year between 1972 and 1999, counting the peregrine and gyrfalcon nests they saw there.

They watched a remarkable recovery. When the elder Burnham and his colleagues first visited western Greenland, in 1972, they found eight peregrine nests and considered the local population healthy. (At that time there were zero peregrine nests in the eastern United States.) In 1999, the year that peregrines were removed from the endangered species list, the Burnhams found more than 170 nests across the island.

Now, thanks to warmer summers, the bird’s profile in Greenland is changing again. In 2016, Burnham’s team estimated there were at least 11 pairs of peregrines north of the American air base in Thule. One pair lives more than 150 miles north of the base, and only about 800 miles from the North Pole.

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.”

A large nest above a lake near Sillisit, Greenland (Robinson Meyer / The Atlantic)

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.

All that water, all at once, can pool in peregrine nests, chilling or even drowning the chicks. “For small, little baby birds, covered in down, the one thing you don’t want is a bunch of rain. It used to be a lot of dry snow that came down, now it’s rain,” Burnham says.

Over time, he thinks these large rainstorms could disturb the bird’s delicate demographics. Peregrines generally lay four eggs in a year, and while all four will usually hatch, one or two chicks will die in the nest. If that chick mortality rate rises over time, it could start to curtail the falcon’s population—and its northward expansion.

“No matter how warm it gets, the falcons may not be able to produce enough young to move the population further north,” Burnham told me. Maybe peregrines are just like us: Right now, they’re enjoying all the good parts of global warming. The suffering will come later.