Birds are getting bigger to survive harsh storms related to climate change, scientists have suggested.
A study spanning 40 years found that birds in central California are significantly larger than they were 25 to 40 years ago.
Robins, for example, have increased by an eighth of an inch in wing length and about 0.2 ounces in weight since the 1980s.
On average, birds in central California have bulked up between two and five per cent in body weight and wingspan.
Researchers suggested this increase in body weight is to protect the birds during increasingly erratic weather as global temperatures rise.
The findings, published in Global Change Biology, are at odds with previous research.
In keeping with this reasoning, birds and mammals would get smaller to adapt to rising global temperatures.
The researchers, led by San Francisco State University graduate student Rae Goodman, wrote: ‘Previous studies from other regions of the world have documented decreases in avian body size and have used Bergmann's rule and increases in mean temperature to explain these shifts.
'Because our results do not support this pattern, we propose that rather than responding to increasing mean temperatures, avian body size in central California may be influenced by changing climatic variability.’
The research suggests the explanation for the birds’ growth is more complex, according to researcher Jill Demers from the San Francisco Bay Bird Observatory.
'The degree of physical change over a relatively short scale of time is remarkable and surprising,’ Demers told the Los Angeles Times.
'Similar studies in Pennsylvania and Europe, for example, show that birds there have decreased in size over the past several decades.’
The trend was discovered by graduate students at San Francisco State University, who analysed data from thousands of birds caught and released each year in California.
The data was gathered from ‘banding stations’ where dozens of species of birds each year are captured, banded around the leg with an identification tag, weighed and measured before being released.
In total, they analysed data from 14,735 individual birds collected from 1971 to 2010 near the southern end of Point Reyes National Seashore, and 18,052 birds collected between 1983 and 2009 from the southern end of San Francisco Bay.
More study is needed to determine the consequences of these changes, Goodman said.
The findings are at odds with research from the National University of Singapore last month, which found birds and mammals are shrinking because of global warming.
Researchers suggested warmer temperatures over the past few years have made some animals adapt to need less body fat to survive while others are struggling for food.
Among the animals which have already started shrinking are Soay sheep, found in Scotland, which were shown in a study to have decreased in size by five per cent between 1985 and 2007 due to warmer winters.
Dr David Bickford from the National University of Singapore reviewed research showing creatures from deer to tortoises, gulls, goshawks and even polar bears have already shrunk over the past 50 years.