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.

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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.

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.

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“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.”