Something weird is going on with the world's smallest birds

Birds live in the background of our lives and humans rarely take note of our feathered friends unless they're sneakily stealing our lunches or pooping on our cars. But something very wrong has been happening to the world's bird populations.

Birds from all over the world are experiencing a shrinkage. However, it is not their populations that are shrinking but rather their body sizes. Birds are getting smaller but it's also a lot more complicated than that.

A group of Yale University researchers discovered that smaller birds are actually shrinking in size a lot faster than their larger counterparts. You might be wondering why, and luckily for you, the researchers were able to provide an answer: it's climate change. 

It turns out that climate change is not only warming our world but it's also having an effect on the size of birds. This might sound really weird but there's a good scientific explanation as to why the world's warmer weather would affect birds.

Scientists have known since at least 2021 that something weird has been going on with the world's bird populations when a study found a major decrease in their body sizes. 

Published in the journal Science Advances, the 2021 study found that every single one of the 77 different Amazonian bird species they looked at over a four-decade period had a lower mean mass, meaning they had shrunk in size. 

But the results of the research weren’t as clear-cut as they sounded, though. While the bodies of the birds studied may have gotten smaller, the researchers found their wing spans had actually got larger. 

The reasons why the birds’ bodies had gotten smaller and their wings had gotten bigger were initially varied but the researchers eventually concluded that global temperature changes played the most important role in changing bird sizes

"After accounting for other factors, the researchers found that the birds' bodily changes were closely linked to the rising temperatures and shifts in precipitation caused by climate change,” wrote Audubon Magazine’s Lauren Leffler.

Moreover, the study’s authors noted in their abstract that their findings could also be attributed to increased pressures that result from warming, and added that seasonal and long-term bird sizes were a response to climate change.

One explanation put forth by the study’s authors was that the changes in the birds they studied may have been due to Bergmann’s rule, which Lauren Leffler defined in detail in her article explaining the 2021 study. 

“In biology, Bergmann’s rule posits that warm-blooded animals are larger in colder climates and smaller in warmer ones,” Leffler wrote. “That’s because larger bodies conserve heat, while smaller bodies disperse it more quickly.”

"Bergmann’s rule also explains why a small cup of coffee cools down faster than a large one: Smaller objects have a higher ratio of surface area to volume, which means more contact with cooler air that absorbs heat," Leffler added, and that might explain why wingspans increased. 

The 2021 study's authors noted that 76% of the birds they looked at fit into Bergmann's rule, especially those that were considered sedentary. But the rule didn’t explain everything. 

Scientists from Yale wanted to investigate these changes in the sizes of today’s birds and looked at the same four-decade period of information as the 2021 study. They also examined an unrelated data set in order to look at bird body size changes in several other species of birds.

By analyzing both sets of data the Yale researchers were able to show that there were changes in 129 different species they studied, which is also how they discovered the shocking information that smaller birds were shrinking faster than larger birds. 

The researchers weren’t able to explain exactly why small birds were shrinking faster than their larger counterparts but they were able to rule out a hypothesis that a particular species' generational lifespan played a role in their changing body sizes. 

However, the researchers did conclude that body sizes in bird populations appeared to be the “primary mediator” in a species' response to climate change. They also added that this should be taken into consideration when trying to understand what’s changing bird sizes.