Why Are Birds Shrinking?

The bodies of birds are shrinking even as their wingspans increase, according to a new study from the USA. The report, published in the journal Ecology Letters, examined over 70,000 bird samples from a multitude of different species and concluded that climate change was responsible for the physiological change in the animals.

The study is the largest of its kind and is one of the first to measure how climate change impacts upon the bodies of animals, as opposed to their migratory and lifestyle habits. Its authors claim that it provides valuable insight into how global warming is having unforeseen consequences for fauna all across the globe.

“Shocking consistency”

The scientific community has demonstrated reluctance to use animals in laboratory settings in recent years, with concerns over animal welfare at an all-time high. However, all 70,716 specimens used in the recent study were already dead before being involved in the research. They were collected by ornithologist Dave Willard, who works at the Field Museum in Chicago. Over a period of almost 40 years, Willard gathered, measured and stored the birds after they collided with the building and died on impact.

When Willard began that process, he did not have any such specific study in mind, but merely believed the findings could prove useful at some point in the future. However, comparison of the various samples showed that the length of the lower leg bone in almost all of the 52 species tested had decreased by 2.4% between 1978 to 2016. Over the same period, the wingspan had increased by 1.3%. “We found almost all of the species were getting smaller," explained Brian Weeks, lead author on the study. “The species were pretty diverse, but responding in a similar way. The consistency was shocking.”

Climate change the culprit

Weeks and Willard, who co-authored the paper, believe that warming temperatures around the world are the reason for the change in the birds’ size. Although the exact explanation is unknown, it’s thought that those of a smaller stature have a larger surface area-to-volume ratio, which allows them to cool off and shed excess heat more quickly. This, in turn, means they are better equipped to handle the heightened temperatures found all over the globe due to the onset of manmade climate change. Meanwhile, those with a longer wingspan are also more capable of handling the arduous journeys involved in migration, meaning they have evolved that way.

Given that the majority of existing studies on the subject revolve around how climate changes impacts human life, the American research provides interesting insight into previously unexplored implications for the avian population. It adds to previous evidence which points to a link between shrinking body sizes in animals. In 2014, two separate studies revealed that both mountain goats and salamanders were decreasing in size as a natural response to global warming.