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Birds in the Amazon have been shrinking. Here's why scientists think it's happening

AILSA CHANG, HOST:

In recent years, scientists found something strange was happening among sensitive bird species in the Brazilian Amazon. Not only were the birds declining in number, but their bodies were shrinking in size.

VITEK JIRINEC: But then we found that size is not only shrinking for those sensitive species. It was declining for everyone.

MARY LOUISE KELLY, HOST:

That is Vitek Jirinec of Louisiana State University. He and his colleagues have now found that over the past four decades, 77 different species of Amazonian birds have been getting slightly smaller on average. Philip Stouffer was Jirinec's advisor at Louisiana State.

PHILIP STOUFFER: 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.

CHANG: Now, if you're wondering why that is, well, the short answer may be climate change. Over the 40-year study period, the rainforest has gotten warmer.

KELLY: That's right. And a smaller bird would shed heat more efficiently. It has more surface area in relation to its volume. Now, here's an example that might be a little easier to relate to.

BRIAN WEEKS: You could imagine lots of little ice cubes in a glass of water, as opposed to one big ice cube, and the little ice cubes melt faster because smaller things have larger surface area-to-volume ratios, so they exchange heat more quickly.

CHANG: Brian Weeks of the University of Michigan didn't work on this particular study, but he did study the size of more than 50 species of migratory birds in North America a few years back. And he found the same thing. Nearly all of them were shrinking decade by decade.

KELLY: The two studies reinforce the idea that birds all over the planet, migratory or not, may be changing shape due to a warming climate. Weeks says these sorts of changes should concern all of us.

WEEKS: All around the world, people depend on natural systems. Intact natural systems provide more economic benefits to humanity than the entirety of the world's GDP, so they matter to you whether or not you know it.

CHANG: Vitek Jirinec of Louisiana State says the timing of his paper's publication could not be more fitting.

JIRINEC: Our study comes out on the same day as the conclusion of the U.N. Climate Change Conference in Glasgow. So those results really underscored the pervasive consequences of our actions for the planet.

CHANG: The study is out today in the journal Science Advances. Transcript provided by NPR, Copyright NPR.