Climate change is shrinking many Amazonian birds

The Amazon rainforest is as pristine a place as most people can imagine, but even there, the effects of a changing climate are playing out. Now, research suggests that many of the region’s most sensitive bird species are starting to evolve in response to warming.

Previous research found that some birds in the undisturbed Amazon—the world’s largest rainforest—are experiencing declines that may be related to climate change. The new study examined four decades of data on bird species and found that as the Amazon’s dry season has gotten hotter and more arid, some species appear to be changing physically.

Birds are often considered sentinel species—meaning that they indicate the overall health of an ecosystem—so scientists are particularly interested in how they’re responding to climate change. In general, the news has not been good. For instance, a 2019 report by the National Audubon Society found that more than two-thirds of North America’s bird species will be vulnerable to extinction by 2100 if warming trends continue on their current course. (Read why birds matter.)

For the new study, researchers collected the biggest dataset so far on the Amazon’s resident birds, representing 77 non-migratory species and spanning the 40 years from 1979 to 2019. They report on November 12 in the journal Science Advances that 36 species have lost substantial weight, as much as 2 percent of their body weight per decade since 1980. Meanwhile, all the species showed some decrease in average body mass, while a third grew longer wings.

During the study period, the average temperature in the region rose, while precipitation declined. Temperatures increased by one degree Celsius during the wet season and 1.65° Celsius in the dry season. Precipitation increased by 13 percent during the rainy season but decreased by 15 percent in the dry season, making for a hotter, dryer climate overall.

These climatic changes overlapped with changes in the birds’ builds, the researchers say, with the dryer climate going further to explain the changes.

“This is a valuable and fascinating paper based on 40 years of data, pretty much unheard of in the tropics,” says Cagan Sekercioglu, a National Geographic Explorer, photographer, and ornithologist at the University of Utah School of Biological Sciences, in Salt Lake City, via email.

Because of the study’s “long time series and large sample sizes, the authors were able to show the morphological effects of climate change on resident tropical birds,” said Sekercioglu, who wasn’t involved in the study.

From fighters to gliders

Researchers focused on bird species that don’t migrate, which ruled out factors such as exposure to different habitats as the cause of any physical changes. The birds in the study spend their whole lives in the undisturbed rainforest understory, just below the tree canopy, so habitat degradation doesn’t factor in, either.

So why would birds be evolving smaller bodies and longer wings?

The researchers themselves are unsure what advantage the wing length changes give the birds, but smaller birds may have an easier time keeping cool. In general, smaller animals have a larger ratio of surface area to body size, so they dissipate more heat faster than a bigger animal. Less available food, such as fruit or insects, in dryer weather might lead to smaller body size. (Learn how climate change is impacting North American birds.)

A comparison to aircraft may offer one explanation for the longer wings, says ecologist Vitek Jirinec, of the Integral Ecology Research Center, in Blue Lake, California, who led the new study.

“Think about a fighter jet,” Jirinec says. It has short wings and is heavy. “It has to go really fast to stay aloft, so it spends plenty of energy,” he says, whereas “a glider almost uses no power to stay aloft, because it’s got these long wings, and it’s light.”

Sekercioglu found the effect on wing length surprising, noting that it underscores the need for more studies in other tropical areas to understand how and why resident birds are responding to climate change with longer wings.

The results also echo findings for near-ground and ground-dwelling rainforest birds in a paper published in October 2020 in Ecology Letters led by avian ecologist Philip Stouffer of Louisiana State University, in Baton Rouge, who has been researching birds in the Amazon since 1991.

Stouffer and most of the study team are part of the Smithsonian Institution’s Biological Dynamics of Forest Fragments Project, a project studying the fragmentation of the rainforest environment. In 2008, Stouffer and his students noticed that they weren’t seeing the same birds they’d seen in previous years of data collection. They decided to start collecting the same type of data that had been collected in the 1980s, allowing them to compare measurements of temperature, precipitation, and bird populations over time.

The work by Jirinec and colleagues “is one of a handful of studies that examined the effects of climate change on tropical resident birds,” Sekercioglu says.

“Because most of the world’s ornithologists and research funding is in developed countries that are mostly temperate, there is much less research on tropical resident birds that comprise the large majority of the world’s bird species,” Sekercioglu says.

In one of the few other long-term studies of tropical birds, Sekercioglu’s work with colleague Bill Newmark, research curator and conservation biologist at the Natural History Museum of Utah, in Salt Lake City, reviewed 30 years’ worth of data on 22 species and found that as temperatures rose, population growth declined.

The other story in the understory

Though most of us probably think of showy macaws and other colorful species as representative of rainforest birds, most are “rather nondescript, drab-plumaged antpittas, antthrushes, antbirds, and leaftossers,” Jirinec says. These birds are “the real symbols of undisturbed Amazonia, because they are highly sensitive to forest disturbance.” (Learn how some species can adapt to climate change—just not fast enough.)

The antpittas, for example, “look like an egg on stilts,” he says, and the musician wren, a tiny, earth-toned puffball with a rich, haunting song are among the birds that have developed longer wings and smaller bodies over generations.

For those who wonder why a small body change in a small, sensitive homebody bird should matter, Jirinec points to how our actions have consequences we don’t always see—such as changing the size and shape of animals half a world away.

“We think of Amazonia as a symbol of terrestrial biodiversity, a mysterious place, teeming with life, untouched by people, away from deforestation,” Jirinec says. “But it looks like no, not necessarily.”