Climate change is shrinking birds in the US

James Marshall
December 6, 2019

As the climate warms, birds are shrinking and their wingspans are growing, according to a new study. Is this a taste of things to come - for humans as well as for birds?

"When we began collecting the data analyzed in this study, we were addressing a few simple questions about year-to-year and season-to-season variation in birds", says Willard, now a collections manager emeritus at the Field Museum.

This isn't the only way that climate change has changed bird ecology recently. Earlier this year, a study from the University of Cape Town found mountain wagtails, a type of bird native to sub-Saharan Africa, have shrunk in average weight over a 23-year period, which the researchers attributed to warming temperatures. In a press statement, he comments: "I was incredibly surprised that all of these species are responding in such similar ways". The largest investigation of its kind, the team carefully analyzed data from the birds that were measured using calipers and scales by Willard who recorded all of his data by hand. The consistency was shocking.

"We found nearly all of the species were getting smaller", said lead author Brian Weeks, an assistant professor at the school for environment and sustainability at the University of MI. Wingspan went up by 1.3% on average, and the two parameters were inversely correlated. The birds with longer wingspans were most likely to survive the long journey as they showed signs of adapting.

We found nearly all of the species were getting smaller.

Published in the journal Ecology Letters, the team outline how increases in temperature associated with climate change are predicted to cause reductions in body size across a range of animal species with the data from the birds being a key demonstrator of this trend. This biological theory is known as Bergmann's rule. Reduced body size means that birds have less energy to complete long and taxing migrations, Weeks tells the BBC's Vlamis. They found an inverse relationship between body size and summer temperature. However, the measurements can not lie, and this study, the largest in its category, shows "the most consistent large-scale responses for a diverse group of birds", according to the researchers. "Being able to show that kind of detail in a morphological study is unique to our study, as far as I know, and it's entirely due to the quality of the dataset that David Willard generated".

This study provides solid evidence of shrinking body size. One possibility they are exploring is called developmental plasticity - how an animal changes its developmental pattern with its changing habitat or environmental conditions.

While most studies that consider animal response to climate change tend to be concentrated on factors associated with geographical range, migration, and population this study proposes that body morphology should also be considered a crucial element.

It is not yet clear whether these changes are harmful to birds, but the study suggests that animal size and shape are an important consideration when predicting how species will react to a continually warming planet.

A study published Wednesday involving 70.716 birds killed from 1978 to 2016 in these collisions in the third largest United States city found that their average body size has steadily declined during this period, although their spans have increased.

Other reports by Click Lancashire

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