In a remote corner of Brazil’s Amazon rainforest, researchers have spent decades capturing and measuring birds in a pristine forest untouched by roads or deforestation. This example of the dazzling diversity of the Amazon was supposed to be a starting point to reveal how birds die because of human activity. But in this pristine region, another, even stranger change is happening: the birds are shrinking.
In 40 years, dozens of species of Amazon birds have shrunk in size. Many species lose nearly 2% of their average body weight each decade, scientists say in an article in Science Advances. The change in other birds is expressed in longer wings. The changes coincide with a hotter and changeable climate, which could give an advantage to weaker, more efficient bodies, which helps birds not to raise their body temperature, the researchers say. “Climate change is not something of the future. This is happening now and there are effects we didn’t think about, “said Ben Winger, an ornithologist at the University of Michigan at Ann Arbor who did not participate in the study but documented a similar shrinkage in the size of migratory birds. Biologists have long linked body size to temperature. In colder climates, it is worth being large and massive, because the smaller surface area reduces heat loss through the skin and keeps the body warmer. As the climate warms, “expect a reduction in body size to help organisms dissipate heat better,” said Vitek Jirinets, an ecologist at the Integral Center for Environmental Research in Blue Lake, California. Many species of migratory birds in North America are becoming smaller, Winger and colleagues reported in 2020 in Ecology Letters. The probable culprit is climate change, Winger said, but because migrations are subject to a wide range of conditions as they travel around the globe, other factors, such as destroyed habitats, cannot be ruled out.
To see if the birds that remain in place are also shrinking, Jirinec and colleagues analyzed data on non-migratory birds collected from 1979 to 2019 in the pristine region of the Amazon, which stretches for 43 kilometers. The data set includes measurements of the mass and length of the wings for over 11,000 individual birds of 77 species. The researchers found that all species declined in mass during this period. The species loses from about 0.1% to almost 2% of its average body weight each decade. These changes coincide with the overall increase in average temperature of 1 degree Celsius in the wet season and 1.65 degrees Celsius in the dry season.