Climate change is affecting the bodies of birds, even in the most remote parts of the Amazon, devoid of direct contact with human beings.
Birds in the Amazon rainforest have become smaller and their wings have become longer over several generations, due to changes in their environment.
"Even in the middle of this pristine Amazon rainforest, we are seeing the global effects of climate change caused by people, including us," said Vitek Jirinec, Louisiana State University (LSU) alumni associate ecologist at the Integral Ecology Research Center.
This is the first study to discover these changes in non-migratory birds' body size and shape.
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Jirinec and colleagues studied data collected on more than 15,000 individual birds that were captured, measured, weighed, marked with a leg band and released, over 40 years of field work in the world's largest rainforest.
The data reveal that nearly all of the birds' bodies have reduced in mass, or become lighter, since the 1980s.
Most of the bird species lost on average about 2% of their body weight every decade.
For an average bird species that weighed about 30 grams in the 1980s, the population now averages about 27.6 grams.
Co-author Philip Stouffer, Lee F Mason professor in the LSU School of Renewable Natural Resources, said: "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.
The data set covers a large range of the rainforest so the changes in the birds' bodies and wings across communities are not tied to one specific site, which means that the phenomenon is pervasive.
"This is undoubtedly happening all over and probably not just with birds," Stouffer said. "If you look out your window, and consider what you're seeing out there, the conditions are not what they were 40 years ago and it's very likely plants and animals are responding to those changes as well.
“We have this idea that the things we see are fixed in time, but if these birds aren't fixed in time, that may not be true."
The scientists investigated 77 species of rainforest birds that live from the cool, dark forest floor to the warmer, sunlit midstory.
They discovered that the birds that reside in the highest section of the midstory and are the most exposed to heat and drier conditions, had the most dramatic change in body weight and wing size.
These birds also tend to fly more than the birds that live on the forest floor. The idea is that these birds have adapted to a hotter, drier climate by reducing their wing loading therefore becoming more energy efficient in flight.
Stouffer said: "There may be other researchers in other places who have relevant data from the 1970s and 1980s that could be compared to modern data, because the bird banding protocol we used is pretty standard.
"So if you measure mass and wing, maybe there will be more datasets that will emerge and we'll be able to get more of an idea of the variation across space and how it might be changing in different systems."
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