Birds have been steadily shrinking in size over the last four decades thanks to the increases in temperatures due to climate change, a study at the Field Museum in Chicago has found.


All of the 52 species of bird in the study showed reduced body size and mass over the forty years of records, with an average 2.4 per cent decrease in leg bone length. However, the birds' wingspans increased by an average 1.3 per cent, which the researchers believe allows the birds to continue making long migrations despite having a smaller body to produce the energy needed for flight.

“When we began collecting the data analysed in this study, we were addressing a few simple questions about year-to-year and season-to-season variation in birds,” said Dave Willard, a collections manager at Chicago’s Field Museum.

“The phrase 'climate change' as a modern phenomenon was barely on the horizon. The results of this study highlight how essential long-term data sets are for identifying and analysing trends caused by changes in our environment.”

Birds' size is related to the temperature of their climate. Birds tend to be larger in cooler climates, since a larger body mass can help them to stay warm. The researchers say the decrease in size is in keeping with the gradual warming of temperatures in Chicago, where the study was carried out.

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The study began by chance in 1978 when Willard heard colleagues that birds were crashing into sides of the huge McCormick Place building, a mile down the road from his job at the Field Museum.

“I was curious, so I went for a walk around the building one morning,” he said. “I found a couple dead birds and I brought them back to the museum – I've always wondered if there had been no birds that morning whether I would have ever bothered to go back.”

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From that day on, scientists and volunteers at the museum collected the birds that collided with the building, taking their measurements and logging them, by hand, in a ledger.


Though the team noticed slight changes in measurements over the years, it wasn’t until the full, four-decade analysis that they realised the extent of the shrinking affect.


Amy ArthurEditorial Assistant, BBC Science Focus

Amy is the Editorial Assistant at BBC Science Focus. Her BA degree specialised in science publishing and she has been working as a journalist since graduating in 2018. In 2020, Amy was named Editorial Assistant of the Year by the British Society of Magazine Editors. She looks after all things books, culture and media. Her interests range from natural history and wildlife, to women in STEM and accessibility tech.