In the heart of the Amazon, where pristine rainforest remains largely untouched by humans, birds are shrinking.
For about four decades, researchers collected and measured 77 species of birds at forest camps. They are not the tie-dye-colored parrots many associate with the Amazon. Instead, they’re mostly nondescript birds that researchers described as “drab” and “brown.”
But in a study published Friday in the journal Science Advances, they’re telling a compelling story — about us, scientists say.
The research suggests that human-caused climate change is piercing even areas considered otherwise largely untouched by people — free of busy roads, pollution and deforestation — and prompting profound changes to the creatures that live there. Every species, on average, weighed less than it had when it was measured in the early 1980s, which the researchers say has no other explanation than a changing environment.
“This study is an example of climate change — human actions globally — affecting a fundamental thing such as the size and shape of these birds in the middle of the Amazon, the symbol of terrestrial biodiversity,” said a lead author on the paper, Vitek Jirinec, a research ecologist.
Birds around the world face climate threats. The U.S. and Canada have lost a quarter of their combined bird populations since 1970, in part because of climate change, a 2019 study found. And unlike many small mammals, which are often nocturnal and able to buffer themselves from temperatures in burrows, nests or caves, birds are constantly exposed to ambient climate conditions, said Blair Wolf, a professor of biology at the University of New Mexico who wasn’t involved in the Amazon research.
That means small birds are often good proxies for the effects of climate change.
The rainforest camps where researchers studied, about 40 miles north of Manaus, Brazil, don’t offer easy access.
Chainsaws are often needed to clear trees from the rough dirt roads that lead into that corner of Earth’s most biodiverse region. Researchers typically spend two weeks at a time camping and studying the lush landscape, away from civilization and its ecological effects.
To capture birds, researchers strung up 6-foot-tall mist nets — mesh that’s invisible to the creatures — across about 650 feet of forest. Then, they’d measure, document and release the species that flew in and were temporarily captured.
It’s a unique setting near the equator that varies little in elevation and is within a connected forest. Researchers have previously observed that migratory birds were shrinking. The Amazon study, for the first time, saw the trend in birds that don’t migrate or move much at all.
The researchers collected data on more than 15,000 birds. Little changed around them — except the climate.
Wet season temperatures have increased by an average of about 1.8 degrees Fahrenheit, with 13 percent more rain, the study says. Dry season temperatures rose by nearly 3 degrees, and rainfall dropped about 15 percent.
In track with the environmental changes, birds' bodies transformed subtly over time. All 77 species studied, on average, weighed less than they did four decades ago. The trend was statistically significant in 44 of the species.
In 61 species, birds’ wings grew longer on average. Nearly a third of the 77 species showed clear trends toward longer wings.
The research was made possible by extensive, detailed data captured over a long time.
“It’s a nice study that’s sort of unparalleled with the amount of data they have,” Wolf said. “This is impressive that they’re seeing a change on a decadal basis based on these background environmental changes in a place that’s been pristine and untouched for a long time.”
The researchers’ data showed links among temperature trends, precipitation trends and body sizes.
Now, the question is: What, precisely, is happening and why?
It’s not clear whether the species’ genetics are evolving to adapt to a new climate.
“Evolution could happen on this time scale,” said a co-author of the study, Phil Stouffer, a professor at the Louisiana State University School of Renewable Natural Resources. “It’s a really important question to understand.”
It’s also unclear what benefit the birds get from the changes to their bodies.
Smaller bodies with bigger wings are more efficient for flying, and the scientists speculated that that could be driving the changes in birds’ bodies.
“You don’t have to flap as fast,” said Jirinec, the study’s lead author. That means birds produce less heat and use less energy to find food.
Wolf, who studies how animals interact with their environment, said that in a hotter dry season with less moisture, being smaller “certainly reduces the cost of living.” Less movement means birds lose less water.
Jirinec said, “The dry season could be that period of time when things could be stressful.”
Precipitation changes, which were more closely linked to the changes in size, could also limit the availability of food and water, causing body changes.
Previous research in this corner of the Amazon shows that bird species has become less abundant over time and in parts of the rainforest that remained intact.
A transformation in body size could help the birds cope with warmer temperatures and changes in precipitation, but it could also alter what the species feed on or how they interact with the environment.
“How small can they get if the climate keeps changing?” Wolf asked. “What are the consequences?”
The paper reinforces that the choices humans make have far-reaching implications for creatures far removed from most people’s daily lives.