A recent study has revealed that the dwindling size of birds across the globe is primarily due to climate change and shifting temperatures.
However, new research conducted by a team of Yale University scientists reveals that smaller birds are diminishing much more rapidly than their larger counterparts.
So what is happening? Why are birds shrinking, and why are the smallest species most affected?
Since at least 2021, when a study revealed a significant decrease in their body sizes, scientists have known that something strange has been occurring with the world's avian populations.
This study, which was published in the journal “Science Advances”, found that every single one of the 77 Amazonian bird species examined over a period of four decades had a reduced average mass, indicating that they had shrunk in size.
But the research findings were not as conclusive as they appeared. While the bodies of the examined birds may have shrunk, the researchers discovered that their wingspans had grown.
Initial explanations for why bird bodies shrunk and wings grew were diverse, but researchers later concluded that global temperature variations played the most significant role in bird size changes.
Lauren Leffler of “Audubon Magazine” wrote: “After accounting for other factors, the researchers found that the birds' physical changes were closely linked to the rising temperatures and shifts in precipitation caused by climate change.”
In addition, the authors of the study noted in their abstract that their findings could also be attributed to the increased pressures caused by global warming, and they added that seasonal and long-term bird sizes were a consequence of climate change.
Bergmann's rule, which Lauren Leffler defined in detail in her article explaining the 2021 study, was proposed by the study's authors as a possible explanation for the observed alterations in the birds they examined.
Leffler wrote that according to Bergmann's rule, warm-blooded animals are larger in colder climates and smaller in milder ones. Larger bodies retain heat, whereas smaller bodies dissipate it more rapidly.
“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 this may explain why wingspans grew.
The authors of the 2021 study noted that 76% of the birds they examined met Bergmann's rule, particularly the sedentary species. The rule did not, however, explain everything.
Yale researchers examined the same four-decade data period as the 2021 study, as well as a second, unrelated data set, to examine body size changes in multiple species.
By analysing both sets of data, Yale researchers were able to demonstrate changes in 129 species they had studied, and they made a startling discovery: smaller birds were shrinking at an alarming rate.
The researchers were unable to explain why smaller birds were diminishing more rapidly than their larger counterparts, but they were able to disprove the hypothesis that the generational lifespan of a particular species played a role in their changing body sizes.
The researchers concluded that the body size of bird populations appeared to be the “primary mediator” in a species' response to climate change, and added that this should be taken into account when attempting to comprehend what was influencing avian size changes.
Environment