As Andes Warm, Deciphering The Future for Tropical Birds by Daniel Grossman: Yale Environment 360

Scientists have theorized that tropical birds in mountainous regions will move uphill as the climate warms. But new research in the Peruvian Andes suggests that the birds will stay put and face a new threat — predator snakes that will climb into their territory to escape the heat.


Gustavo Londoño squats motionlessly under a rainforest canopy in Peru’s Manú National Park, staring into the dense undergrowth. A dove coos somewhere. An oropendola bird warbles. Birds must be out there amid the tree crooks, upturned roots, and underground cavities. But where?

Daniel Grossman is a journalist and radio and web producer who has reported from all seven continents. He is author of Deep Water: As Polar Ice Melts, Scientists Debate How High Our Oceans Will Rise. Previously for Yale Environment 360, Grossman reported on the impacts of melting glaciers on two Himalayan towns. His work on this article was supported by the Pulitzer Center on Crisis Reporting.


Londoño, a Colombian biologist, has been searching painstakingly to find their nests. He’s located hundreds of them here on the Amazon flank of the Andes, though it’s taken him eight years. He has rigged each one with monitoring gear to collect data that will help him better understand how tropical birds will respond to global warming. 

Climate change is expected to warm this region about 7 to 11 degrees Fahrenheit in this century. Researchers have predicted that the warmer temperatures would force birds uphill, wreaking havoc on them and their ecosystems. But Londoño thinks they’ll stay put — and face a different kind of existential threat. 

Now a professor at Universidad ICESI in Colombia, Londoño began these studies in 2007, while in graduate school at the University of Florida. He set off then to Peru to study an issue that has intrigued scientists at least since the 18th century, when pioneering biologist Carl Linnaeus published his 1781 treatise On the Increase of the Habitable Earth: Why do plants and animals segregate themselves by altitude? Linnaeus believed that species had spread from mountaintops to lowlands after the the biblical flood receded. Modern scientists discarded this theory long ago, but they are still groping for a completely satisfactory alternative. 

This segregation by altitude is especially noticeable in mountainous regions, such as Manú National Park, where Peru’s national bird, the cock-of-the-rock, lives between 3,000 and 7,500 feet above sea level, and nowhere else. The hooded mountain tanager lives only at between 7,900 and 10,500 feet. Indeed, this Connecticut-sized mountainous region is home to more species of birds than all of the contiguous United States, many sorted in band-like territories. 

Temperatures in the tropics vary little — generally only a few degrees — over a day and between months. (By contrast, in a Northeast U.S. city such as Boston, temperature fluctuates from a low near zero to the 90s most years, a span of about 90 degrees Fahrenheit.) For this reason, in 1967, Daniel Janzen, a young biology professor at the University of Kansas, published a paper, now considered a scientific classic, in the well-regarded journal The American Naturalist hypothesizing that temperature plays a potent part in anchoring plants and animals to specific elevation zones in the tropics. Janzen theorized that, after thousands of years of stable tropical conditions, plants and animals couldn’t adapt to big temperature shifts. This hypersensitivity, he said, restricted each species to a limited range of altitudes, as temperature correlates closely with elevation — around the globe, temperature shifts roughly 1.5 degrees Fahrenheit with every 500 feet of altitude. 

Until recently, little research had examined the significance of Janzen’s paper, though biologists generally considered its thrust correct. “It was ignored for 20 years,” says Janzen, now at the University of Pennsylvania. But that changed as researchers refined predictions of climate change’s effects on tropical forests, which are among earth’s most biologically rich places. Climate models generally predict that the tropics will warm less than almost anywhere else on earth. But Janzen’s theory implies that global warming could still cause disproportionate change there. It suggests that mountainous species will migrate uphill to avoid intolerable heat. However, mountains have tops, so animals and plants already at the highest elevations could migrate no farther. Species moving upward would cram into increasingly shared turf. 

Londoño is one of a growing number of scientists whose studies seek to shed light not on whether global warming is happening, but on what ecosystems and species will look like after it takes hold. “Who is going to win here?,” asks Londoño. “Because there’s not enough space for everybody.” 

Even if shared high ground served as a climate-change ark, another problem arises. Species differ in their abilities to pack up and move. Birds and many insects and mammals are highly mobile, but trees migrate only as new generations mature from dispersed seeds. These differences most likely will scramble ecosystems. Predators might drift outside the ranges of their prey. Birds might lose access to nest-building supplies or favored seeds. Londoño says these indirect consequences of warming might turn out to be the most damaging of all.