Climate change has caused migratory birds in North America to shrink in size — while their wings grew longer — over the last 40 years, a study has found.
Researchers from Chicago and Michigan studied more than 70,000 small, migratory song birds that had been collected after fatal building strikes.
Scientists have long known that individuals from a given animal species tend to be smaller in warmer parts of their habitat — a pattern dubbed 'Bergmann's rule'.
Although it had been wondered if climate change might also be triggering shrinking body sizes, evidence to support this hypothesis had been inconclusive until now.
The scale of the collection enabled conclusive proof — even though the changes in bird size and wing length are subtle, at mere grams and millimetres respectively.
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Climate change has caused migratory birds to shrink in size over the last 40 years, a study has found. Pictured, some of the specimens held in the collections of the Field Collection, including an eastern meadowlark, far left, and an indigo bunting, far right
Ornithologist David Willard measured and recorded the body size of 70,716 birds collected after fatal collisions with buildings in Chicago.
He recorded their bill length, wing length and body mass — along with the length of a lower leg bone called the 'tarsus'.
In birds, the length of the tarsus is considered the best measure that represents the variation in body size among a given species.
The team found that — since data gathering started in 1978 — tarsus length declined by around 2.4 per cent on average.
These changes are subtle — reflected in only a few grams difference in body mass — and would not be able to be detected with the naked eye, highlighting the power of collections such as those made by the Field Museum.
A bird from the field museum collection is measured as part of the study
Field Museum ornithologist and collections manager David Willard measured 70,716 birds from 52 species as part of the study, recording their body sizes and shapes.
The collection focuses on small-bodied songbirds, with species of sparrow, warbler and thrush making up the majority of those studied.
The birds themselves were retrieved after fatal collisions with Chicago buildings during both spring and autumn migrations — with such collections been undertaken each year since 1978.
Analysing the vast dataset, researchers found that the body sizes of all 52 species studied have been decreasing over the last four decades — with statistically significant shrinking seen in 49 species.
'We had good reason to expect that increasing temperatures would lead to reductions in body size, based on previous studies,' said paper author and conservation ornithologist Brian Weeks, of the University of Michigan.
'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.'
The team also found that, across the study period, wing-length also increased significantly among 40 species of bird, by an average of 1.3 per cent — with those undergoing to most rapid size reductions also elongating their winds the fastest.
Across the course of the study, temperatures at the birds' summer breeding grounds north of Chicago were seen to increase by around 1.8°F (1°C).
The team believe that the reduction in body sizes is a response this global warming, with the concurrent growth in wing length intend to offset the losses in body mass.
There are several lines of evidence that support this conclusion, they added, with the main being that — alongside the overall trend — the team also found many similar, synchronised fluctuations in both body size and temperature.
'Periods of rapid warming are followed really closely by periods of decline in body size, and vice versa,' Professor Weeks said.
'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.'
University of Michigan ornithologist Benjamin Winger added that it had 'really been a herculean effort [...] to get such valuable data from birds that might otherwise have been discarded after