Are birds getting bigger because of global climate change?


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Birds in central California are significantly larger than they were 25 to 40 years ago, and researchers believe it may be because they are bulking up in body weight to ride out severe storms related to global climate change.

Over the last 25 years, a robin, for example, has increased about an eighth of an inch in wing length and about 0.2 ounces in mass, according to a paper published online in Global Change Biology.


The findings fly in the face of assumptions based on an ecological benchmark known as Bergmann’s rule: Birds and mammals tend to be larger at higher latitudes, perhaps to conserve body heat. Under this reasoning, birds and mammals would get smaller as they adapted to rising global temperatures.

But they also suggest that explanations for the bigger birds are more complex, according to researcher Jill Demers, executive director of the San Francisco Bay Bird Observatory.

“The degree of physical change over a relatively short scale of time is remarkable and surprising,” Demers said. “Similar studies in Pennsylvania and Europe, for example, show that birds there have decreased in size over the past several decades.”

Overall, birds in central California have grown an average of 2% to 5% in body weight and wingspan, said Rae Goodman, who discovered the trend while working as a graduate student at San Francisco State University, analyzing data from thousands of birds caught and released each year near San Francisco Bay and the Point Reyes National Seashore.

More study is needed to determine whether these changes are good for central California birds and how they affect food chains, Goodman said.

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, allowing researchers to analyze physical traits over several decades.


Researchers, including a team from PRBO Conservation Science, analyzed 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.

“I recently presented my research to my students,” said Goodman, who now teaches biology at San Francisco’s Jewish Community High School of the Bay. “It was something a little more glamorous than the lessons they’re used to.”


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-- Louis Sahagun

Photo: Common yellowthroat. Credit: Courtesy of San Francisco Bay Bird Observatory.