Medical News Today: Global warming may cause birds to shrink

As the planet continues to warm, researchers are increasingly pointing out the ways in which global warming will affect our health and that of the species around us.

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Medical News Today have reported on many of these studies.

For example, some species of fungi that are resistant to existing treatments may be on the rise as a result of global warming, one study showed.

Also, several species of fish may become more toxic as a result of warming waters, and malaria may spread to an unprecedented extent.

Now, new research appearing in the journal Ecology Letters explores the ways in which birds may have been affected by rising global temperatures.

Benjamin Winger, Ph.D., of the Department of Ecology and Evolutionary Biology and the Museum of Zoology at the University of Michigan, in Ann Arbor, is the senior author of the new study.

Studying over 70,000 birds

As Winger and colleagues explain in their paper, existing research supports the idea that increasing global temperatures will cause reductions in animals’ sizes.

In fact, a premise in ecogeography known as Bergmann’s rule states that animals tend to be smaller in warmer parts of the world, compared with their counterparts of the same species that live in colder climates.

To determine whether this effect is already noticeable in birds as a result of global warming, the researchers analyzed 70,716 dead migratory birds of 52 North American species.

The researchers obtained these birds from the Field Museum of Natural History, in Chicago, IL, which had been collecting them from building collisions since 1978. So, Winger and the team were able to examine changes in the birds’ body sizes over a 38-year period ending in 2016.

The scientists measured and analyzed the sizes of a lower leg bone called the tarsus, as well as bill length, wing length, and body mass.

Most of the birds in the analysis were species of sparrow, warbler, or thrush — small songbirds that breed north of Chicago.

Changes in bird size over 40 years

Over the study period, the team found a decrease in body size and shape in all 52 species. Of these, 49 species showed a statistically meaningful decrease.

More specifically, tarsus length decreased by 2.4% across all species. The researchers also noted a mean increase of 1.3% in wing length.

Furthermore, they found that species with the fastest reduction in tarsus length also showed the fastest increase in wing length.

Finally, there was a clear relationship between the body size of the birds and the temperature — the higher the temperature, the smaller the body size.

The average temperatures at the birds’ summer breeding grounds had increased by 1.8ºF over the nearly 40-year study period.

Long-term data and surprising results

Brian Weeks, Ph.D., an assistant professor at the University of Michigan School for Environment and Sustainability, is the first author of the paper.

He notes, “We had good reason to expect that increasing temperatures would lead to reductions in body size, based on previous studies.”

“The thing that was shocking was how consistent it was. I was incredibly surprised that all of these species are responding in such similar ways.”

Brian Weeks, Ph.D.

“Periods of rapid warming are followed really closely by periods of decline in body size, and vice versa,” Weeks explains.

“Being able to show that kind of detail in a morphological study is unique to our paper, as far as I know, and it’s entirely due to the quality of the dataset that David Willard generated,” he adds.

David Willard, Ph.D., is a collections manager emeritus at the Field Museum and a co-author of the study.

Willard says, “The results from this study highlight how essential long-term data sets are for identifying and analyzing trends caused by changes in our environment.”

The team also points out the need to further analyze the mechanisms that may explain their findings. The changes in size and wing length observed in the study may result from a mechanism called developmental plasticity.

The researcher continued, “It’s clear that there’s a third component — changes in body size and shape — that’s probably going to interact with changes in range and changes in timing to determine how effectively a species can respond to climate change.”