The results of a study on birds in the Amazon rainforest has shown that birds have gotten physically smaller over the last four decades, and scientists believe that a warming planet may be the reason.
“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,” ecologist Vitek Jirinec told NBC News. Jirinec was a lead author on the study, which was published in the journal Science Advances on Friday.
The paper analyzed measurements and weigh-ins of 77 different bird species since the early 1980s. Nearly all of the bird species have gotten lighter as time goes on. On average, most species became about 2% lighter every decade, which is significant for such small creatures.
“These birds don’t vary that much in size,” co-author Philip Stouffer, a professor of conservation biology at Louisiana State University, said in a release from the institution. “They are fairly fine-tuned, so when everyone in the population is a couple of grams smaller, it’s significant.”
While birds’ bodies have gotten smaller over time, their wings have gotten longer. The researchers believe this may be an adaptation to hotter temperatures, since longer wings and a smaller bodies mean that birds have to use less energy to move around, helping them stay cool.
“The thing that is the most striking about this to me is that this is in the middle of the most intact tropical rainforest in the world,” Stouffer told NPR.
The researchers looked at nonmigratory birds ― birds that live within the confines of the rainforest. And the birds they studied lived in an area untouched by threats like deforestation or other development. The only thing changing much around them was the climate: The wet seasons have become wetter while dry seasons have become more dry, and average temperatures have increased in both.
The researchers found that bird species living in higher parts of the forest ― as opposed to closer to the forest floor — saw the biggest changes in terms of body composition. That supports the idea that a changing climate is the cause, since those are the species most exposed to heat. They’re also the species that fly the most, meaning that conserving energy while doing so would be the most important for them.
This isn’t the only study to suggest that climate change has played a role in animals getting smaller. A 2019 study led by University of Michigan researchers analyzed the average sizes of 52 species of migratory birds in North America from 1978 to 2016. All of them had smaller bodies and longer wings in 2016.
And a study published earlier this year found that critically endangered North Atlantic right whales are, on average, about 3 feet shorter than they were 20 years ago. Scientists attributed this to stress caused by myriad human-related causes — among them anthropogenic climate change, which has forced the whales to swim farther north to reach their cold-water prey.