Birds are changing shape in response to climate change, with species from North and South America getting smaller and longer-winged as the planet warms.
US ecologists have analysed body-size and wing-length data from more than 86,000 bird specimens collected over four decades in North and South America.
They found the smallest bird species are changing the fastest.
“Body size appears to be a primary mediator of birds’ responses to contemporary climate change,” says co-author Brian Weeks, an assistant professor at the University of Michigan.
The new analysis published in the Proceedings of the National Academy of Sciences draws on extensive datasets from two previous studies. One examines migrating birds killed after colliding with buildings in Chicago. The other investigates non-migrating birds netted in the Amazon. The data includes 129 bird species: 52 migratory species breeding in North America and 77 South American resident species.
Despite the lack of overlap in bird species and geography in the two studies, bird species in both North and South America showed similar trends over time, with body changes attributed to increasing temperatures over the past 40 years.
The authors say further research is needed to understand why smaller birds are changing faster, and the biological mechanisms driving the shift in size.
“If natural selection plays a role in the patterns we observed, our results suggest that smaller bird species might be evolving faster because they experience stronger selection, are more responsive to selection, or both,” Weeks says.
The authors say their findings raise concerns about extinction risk for larger-bodied birds, given their bodies responded more slowly.
“Body size may be a valuable predictor of adaptive capacity and the extent to which contemporary evolution may reduce risk of extinction among species,” the authors write.