A deep-blue flowerpiercerGraham Montgomery / University of Connecticut

In 1985, John Fitzpatrick hiked up a ridge called Cerro de Pantiacolla, in the Peruvian Andes, in search of birds. On an eight-kilometer uphill walk, he and his team meticulously documented all the birds that lived on the mountainside. They found dozens of species, many with delightfully ostentatious names. The buff-browed foliage-gleaner. The hazel-fronted pygmy-tyrant. The fulvous-breasted flatbill. The variable antshrike. After their census, they returned to their base camp on the banks of the Palatoa River, and Fitzpatrick took a photo of the Andes, towering in the distance. Then they sailed away.

Thirty years later, Benjamin Freeman, an ecologist at the University of British Columbia and a former student of Fitzpatrick’s, decided to retrace the same hike to see whether the birdlife had changed in the intervening decades. “We motored up the river in a canoe, literally holding up that photo, until we said, ‘Oh, we’re here,’” Freeman says.

As they climbed, the thickets of tall bamboo by the river gave way to big rainforest trees with huge buttress roots. And at the top of the ridge, 1,400 meters up, the team found a “gnarled, mossy wonderland of stunted trees,” Freeman says. What they did not find, however, was the buff-browed foliage-gleaner, the hazel-fronted pygmy-tyrant, the fulvous-breasted flatbill, or the variable antshrike. They had all disappeared.

Mountains vary substantially as they rise, in both climate and vegetation, like layered cakes in which every tier is radically different. For that reason, mountains are hotbeds for the birth of new species, many of which tend to stick to a very narrow band of altitude. That’s true even for birds. It’s easy to imagine that, being winged, they can travel wherever they like, but they too are restricted by layers of climate. And those restrictions are changing.

As the planet slowly warms, the cooler, higher layers of mountains are becoming like the warmer ones at lower altitudes. Animals and plants are tracking these changes, moving upslope in search of their ideal climes. Apollo butterflies have moved 200 meters up the faces of Spanish mountains. Pikas—hamsterlike relatives of rabbits—have disappeared from much of California’s Sierra Nevadas, first from the lowest elevations and then eventually from the highest ones, too. In Cerro de Pantiacolla, Freeman calculated that birds would have to move about 75 meters upslope to experience the temperatures they enjoyed back in 1985.

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The reality wasn’t quite that stark. When Freeman and his colleagues compared their two censuses, they realized that the ridge’s birds are now living, on average, 40 meters higher than they used to. But of the 16 species that once lived at the ridge’s summit, eight were nowhere to be seen. It’s as if all the birds in Cerro de Pantiacolla have been slowly moving upwards, and those that were already at the top just ran out of mountain.

Of course, it’s possible that the team just failed to find the missing birds. But five of these species were common back in 1985, and seven have distinctive songs that should have been obvious parts of the dawn chorus. Freeman thinks that at least some of them really are gone—and others will likely follow. The scarlet-breasted fruiteater, deep-blue flowerpiercer, and russet-crowned warbler now live only in the highest 100 meters of the ridge. After another decade of warming, they will probably disappear, too.

As the team wrote, “In the timespan of just one and a half human generations, warming of less than 0.5 degrees Celsius has set in motion an escalator to extinction for Andean birds.”

To clarify, none of the missing species is extinct yet; they also live on other, taller mountains that provide more space into which they can ascend. But what happened in Cerro de Pantiacolla is likely to happen elsewhere, and even the loftiest mountains are finite. In that sense, the ridge’s birds are harbingers of the future—the hazel-fronted pygmy-tyrants in the coal mine.

“For over 30 years, scientists have been concerned that global warming will cause mountaintop extinctions, but until now there has been little evidence,” says Morgan Tingley from the University of Connecticut. Freeman’s study changes that. “It’s terrifying, like a nightmare come true. It may just be one study from one isolated mountain, but it’s still alarming. If this is how climate change will play out across tropical mountains, then we’re in deep trouble.”

In a way, it’s surprising that scientists haven’t detected more of these disappearances before. One study predicted that a global temperature rise of 2.8 degrees Celsius (5 degrees Fahrenheit) would kill off between 400 and 550 bird species, solely by pushing them off the top of the extinction escalator. And yet “there are few examples where people have actually validated that this is happening,” Freeman says, “and it hasn’t been for lack of effort.”

The problem, he says, is that most researchers have looked for mountaintop extinctions in temperate parts of the world, where animals already have to cope with climates that fluctuate wildly with the seasons. “A little bit of warming might not matter that much to them,” Freeman says. But in the tropics, where temperatures are more stable year-round, animals can become more tightly bound to a narrow range of conditions—and more vulnerable when those conditions change.

Freeman first found evidence of this in 2014, when he and his wife, Alexandra Class Freeman, visited two mountains in Papua New Guinea that an ornithologist had studied in the 1960s. Forty years later, most of the birds had moved to higher regions. It’s not entirely clear why this happens. The birds’ bodies could operate best in specific temperatures. Alternatively, “the warming might matter to the insects, and the insects matter to the birds,” Freeman says. Either way, he suspects that the extinction escalator runs far faster in the tropics than it does elsewhere.

“Barring reducing rates of climate change, what can you do?” Freeman asks. The best interim solution is to preserve tracts of forest that ascend up mountainsides—corridors in which species can travel as warming temperatures push them upward.

“On the main Andes, if you protect a big swath of land, you’ll provide the space for the plants and animals to live in as they move up the mountain,” Freeman says. As for smaller mountains like Pantiacolla, “they’re not tall enough to handle a lot of upslope shifts,” he adds. “These places are basically shit out of luck.”