(The Hill) – The forest of Amazonia and the urban canyons of Chicago are dramatically different environments.

But in both, bird life has displayed the same mysterious pattern: While all birds are shrinking, small birds are getting smaller, faster, than larger ones, a new study has found.

At the same time, in a surprising twist, their wings are getting longer — as though all birds were becoming a bit more every year like tree swallows — and scientists have no idea why.

The findings published Monday in Proceedings of the National Academy of Sciences (PNAS) begin with a large and macabre body of evidence. 

Since 1978, scientists at Chicago’s Field Museum have collected the corpses of unlucky birds.

These are a diverse bunch, owing to Chicago’s lethal — if scientifically fortunate — position as a giant city, full of deadly glass windows on a major migratory route for birds across the country.

For nearly 40 years, Field Museum scientists have risen long before dawn every morning of the spring and fall migration period to collect the bodies of birds who have crashed into the windows of city buildings.

By surveying changes in more than 70,000 specimens from 52 species — most of which had crashed into the windows of McCormick Place, North America’s largest convention center — the Field Museum announced in 2019 that birds, in general, were getting smaller.

The magnitude of the changes were subtle enough to have been easily missed. “It’s a matter of millimeters, tenths of millimeters,” Field Museum scientist Dave Willard said at the time. “It’s not something you know is happening until the analysis.”

The researchers hypothesized that the ultimate cause for the shrinking birds was climate change, because smaller bodies release heat more easily.

But that required more evidence that the phenomenon was in fact happening worldwide — a conclusion to which the PNAS study published Monday gives added support.

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To build that study, the Field scientists collaborated with a team studying nonmigratory birds in the Amazon, which had taken measurements from more than 15,000 birds making up 77 species.

Virtually everything about these two studies was different: the species in question, the geography, the birds’ lifestyles and even the way the specimens were taken. (Their fates were different too: Unlike the dead Chicago birds, the Amazonian birds were netted, measured and released.)

But researchers nonetheless found a “remarkable” overlap, as the Amazonian species “also experienced similarly widespread declines in body size with concurrent increases in wing length,” the authors wrote.  

More thought-provoking: Small birds were changing faster than big ones both in terms of smaller body size and greater wing length.

One thing is clear: It’s easier to be small in hot climates, and both the Amazon and North America have gotten consistently hotter as the species’ sizes have declined. 

But that doesn’t explain why small birds are shrinking faster than larger ones. 

One possible explanation the researchers tested was that little birds simply reproduced more quickly, effectively causing evolution to work on them faster, as The Hill reported.

But when scientists controlled for the length between generations — a metric of how fast birds breed — they found that there wasn’t any relationship between the speed of breeding and the rate of shrinking.

The only thing that helped predict how quickly a bird species would shrink was how large (or small) it was to start with.

The core question is whether evolution by natural selection is driving these changes at all, Benjamin Winger, a coauthor from the University of Michigan, told The Hill.

In such a paradigm, the actual genetic code inside the birds is changing, causing them to bear smaller offspring. 

In such a case, “every generation the smaller birds are doing better, producing more offspring, and dying less, there is more selection toward smaller body size,” Winger said.

That’s not the only possibility, Winger said. In some species, adult birds who were incubated under higher temperatures turn out to be smaller, with faster metabolisms, than birds from cooler nests. 

But that theory doesn’t really provide a full explanation either, the researchers concluded. While many birds exhibit such “plasticity” — or changes in bird body size or composition based on changing nest conditions, rather than genetic code — others do not. 

Also, they write, “it is not obvious” why such plasticity would be more of a factor for little birds than big ones.

The question is an important one: The writers noted that rapid evolution may be needed for many species to survive in environments that are also changing rapidly.

Scientists still don’t know what causes some species to evolve faster than others, or what factors those correlate with — but this paper gives them a useful proxy for telling what birds are likely to change the fastest, even if the reason why is still unclear.

To Winger, the findings were a window onto a larger reality. “To me, the bigger picture is the universality of global change, and its effects on species that may live in dramatically different places,” he said.

“The congruence between those two studies is what I find really striking,” Winger added.