Not only were the birds declining in number, but their bodies were also shrinking in size.
"We found that size is not only shrinking for those sensitive species — it was declining for everyone," said researcher Vitek Jirinec of Louisiana State University.
Jirinec's findings are contained in a new study published in the journal Science Advances last Friday.
It was enough to raise alarm bells for Jirinec's supervisor, 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," Stouffer said.
The study examined 77 species over a 40-year period, during which time the rainforest had become warmer. It found they were rapidly evolving — perhaps because smaller birds shed heat more efficiently as they have more surface area in relation to volume.
Brian Weeks of the University of Michigan explained it this way:
"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."
Weeks didn't work on this particular study, but he did research the size of more than 50 species of migratory birds in North America a few years back. He too found that nearly all of them were shrinking decade by decade.
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 said these sorts of changes should concern all of us.
"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," he said.
Jirinec said the timing of his paper's publication could not be more fitting.
"Our study [came] 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," he said.