Alex Fox of Smithsonian writes about the evolution of Amazon birds in response to climate change:
When the first ever World Climate Conference concluded in February 1979, the scientists in attendance issued a statement calling on world leaders "to foresee and prevent potential man-made changes in climate that might be adverse to the well-being of humanity." On October 17 of that same year, scientists deep in the Brazilian Amazon unfurled a set of 16 mist nets at 6 a.m. to begin a study of the birds living in the understory beneath the rainforest’s green roof.
In the 40 years that followed, climate change went from a far-off-seeming idea to a grave reality that grips every square inch of the planet, and hundreds of dedicated researchers kept opening the mist nets at dawn to capture and study the feathered inhabitants of an intact patch of Brazilian rainforest about 40 miles north of Manaus.
Now, a new paper leveraging this long-running study, originally aimed at testing the impacts of forest fragmentation, shows that as human activities have altered Earth’s climate, the bodies of birds living in the understory of this remote, undamaged patch of rainforest have been changing in response. The authors of the paper report today in the journal Science Advances that all 77 species of birds surveyed by the study weigh less on average than they did 40 years ago and nearly 80 percent of those species also have developed greater average wing-lengths.
Researchers aren’t yet sure what the consequences of these physiological changes might be or the precise mechanisms that gave rise to them, but the team’s analyses suggest the rising temperatures and changes in rainfall seen at the study site offer the most powerful statistical explanation for the birds’ transformation.
“This is the middle of the Amazon rainforest, far away from deforestation,” says Vitek Jirinec, an ecologist at Louisiana State University and the paper’s lead author. “But even here, in this place that is teeming with life and looks totally undamaged, you can’t escape the consequences of climate change.”
Jirinec and his co-authors embarked on this study in earnest in 2020 after finding that 21 species of birds at this site north of Manaus, known to researchers as the Biological Dynamics of Forest Fragments Project (BDFFP), were in decline. Even within this protected area, buffered from logging and pollution, some species had declined by as much as 40 percent, especially insect-eaters. Those results, published in 2020, led Jirinec and his colleagues to try to tease out what might be going on, and, in particular, to probe the role of climate change.
To do that, the researchers compiled the weights of 14,842 individual birds and the wing lengths of 11,582 birds recorded by BDFFP scientists between 1979 and 2019 and paired those data with the last 50 years of changes in temperature and precipitation in the region.
In terms of climate change, the team found that compared to 1966 this region’s wet seasons have become 13 percent wetter and its dry seasons are now 15 percent drier. The average temperature for both seasons has also increased over that time span, with temperatures rising by 1.8 degrees Fahrenheit in the wet season and 2.97 degrees in the dry season.
Among the birds, all 77 species in the study showed average decreases in body weight over the last four decades, with some species losing nearly 2 percent of their mass every decade, and 61 species showed increases in average wing-length. Statistical analysis linked those changes to climatic shifts.
The results fall short of demonstrating cause and effect, but show a strong association. “The relationship between body size and climate change is correlational, naturally,” writes Mario Cohn-Haft, an ornithologist with Brazil’s National Institute of Amazonian Research who wasn’t involved in the paper, in an email. “But both several-decade long trends and year to year trends are demonstrated here with a monstrous amount of data to support them.”
The study found that birds tended to be lighter following hotter and drier conditions than usual, especially if those conditions fell during the dry season, which is the most stressful time of year for birds because food is harder to find.