Pecking disorder: Polar bears are shrinking, while some birds are becoming bigger, all thanks to climate change

Polar bears are some of the most famed hunters on the planet. But even these alpha predators are falling at the cruel hands of climate change, without even a chance to claw back.

What follows is that the polar species, currently the largest family of bears on the planet, is shrinking at a rapid rate. Some shocking estimates have suggested that polar bears have shrunk to two-thirds their original size in the past three decades.

Scientists attribute this to the fact that on a dramatically heating planet, there is a natural tendency for animals to get smaller. Further, Bergmann's rule states that larger animals tend to thrive in colder environments due to their better heat retention. This trend, evidenced by our bears, and even other shrinking birds in many parts of the world, led many to believe it would be a universal consequence of climate change.

But in a surprising twist on a familiar story, birds in the Usambara Mountains of Tanzania are getting bigger, not smaller, as temperatures rise. Researchers found that 42 bird species in the region, from the dark-backed weaver to the square-tailed drongo, all increased in body mass by an average of 4.1% over the past 36 years. Notably, smaller species experienced proportionally larger size increases as well.

The reason behind this unexpected growth remains unclear. Researchers propose several possibilities: longer growing seasons leading to more food resources, decreased competition due to population declines, and even the long-term legacy of habitat fragmentation necessitating larger wings for navigating fragmented forests.

This study is significant for several reasons. First, it underscores the importance of studying climate change impacts in diverse regions, not just low-lying areas like the Amazon. The Usambara Mountains, at 3,000 feet above sea level, offer a unique perspective on how mountainous tropical regions might respond to a warming planet.

Second, the findings highlight the need to move beyond simple assumptions about climate change effects. Understanding how birds respond to factors like food, rather than just direct heat tolerance, is crucial for effective conservation efforts.

Finally, knowing the specific factors driving population changes in the Usambara Mountains allows for more targeted conservation approaches. By understanding the causes of bird size increases, researchers can develop strategies to protect these vulnerable species in the face of a changing climate.

The findings of this research have been published in Ecography.