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(CNN)Birds' bodies are shrinking in response to climate change, even in places like the Amazon rainforest that are relatively untouched by human hands.
Researchers have studied information on more than 15,000 non-migratory birds spanning 77 species over a 40-year period that were captured in the Amazon rainforest, tagged and then released.
The scientists found that nearly all of the birds' bodies have become lighter since the 1980s, losing on average about 2% of their body weight every decade, according to a new study published Friday in the journal Science Advances. For an average bird species that weighed about 30 grams (1 ounce) in the 1980s, the population now averages about 27.6 grams (0.97 ounce). The study also revealed that wingspan was getting bigger in a third of the Amazon bird species studied.
It's a pattern that's also been spotted in North American migratory birds.
"These birds don't vary that much in size. They are fairly fine-tuned, so when everyone in the population is a couple of grams smaller, it's significant," said coauthor Philip Stouffer, who is the Lee F. Mason Professor in the Louisiana State University School of Renewable Natural Resources.