Washington: As the climate warms, many birds are shrinking in size while their wingspans are increasing, according to a study.
Researchers from the University of Michigan in the US analysed a dataset of 70,000 North American migratory birds from 52 species that died when they collided with buildings in Chicago, US.
They found that, from 1978 through 2016, body size decreased in all 52 species, with statistically significant declines in 49 species.
The findings, published in the journal Ecology Letters, also show that wing length increased significantly in 40 species over the same period.
"We had good reason to expect that increasing temperatures would lead to reductions in body size, based on previous studies," said lead author of the study Brian Weeks, an assistant professor at 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," Weeks said in a statement.
The new study is the largest specimen-based analysis of body-size responses to recent warming, and it shows the most consistent large-scale responses for a diverse group of birds, Weeks said.
Several lines of evidence suggest a causal relationship between warming temperatures and the observed declines in avian body size, according to the researchers.
The strongest evidence is that - embedded within the long-term trends of declining body size and increasing temperature - there are numerous short-term fluctuations in body size and temperature that appear to be synchronised, they said.
"Periods of rapid warming are followed really closely by periods of decline in body size, and vice versa," Weeks said.
Within animal species, individuals tend to be smaller in warmer parts of their range, a pattern known as Bergmann's rule, the researchers said.
While the possibility of body size reduction in response to present-day global warming has been suggested for decades, evidence supporting the idea remains mixed, they said.
For each bird, the researchers measured the length of a lower leg bone called the tarsus, bill length, wing length, and body mass.
In birds, tarsus length is considered the most precise single measure of within-species variation in body size.
Three measures of body size - tarsus length, body mass and PC1, a common measure of overall body size that combines several key body-part measurements -- showed statistically significant declines.
Tarsus length declined 2.4 per cent across species while wing length showed a mean increase of 1.3 per cent, the researchers said.
Species with the fastest declines in tarsus length also showed the most rapid gains in wing length, they said.
The researchers noted that mean summer temperature was significantly negatively associated with bird body size - meaning that body size decreased significantly as temperatures warmed.