(ISNS) -- Alfred Hitchcock would have appreciated this twist: The birds in central California are getting bigger.
Researchers at San Francisco State University have found that the birds on both ends of San Francisco Bay have greater body mass and longer wingspans since Hitchcock made the 1963 horror film, "The Birds," in which birds went mad and attacked a town on the coast.
Researchers said that the reason for the growth is believed to be climate change, but therein lies a mystery: The birds should be getting smaller as the climate warms, not bigger.
San Francisco State University graduate student Rae Goodman and assistant professor Gretchen LeBuhn used data from 14,735 birds collected since 1971 near the northern tip of the bay at the Palomarin Field Station at Point Reyes National Seashore, and 18,052 birds collected between 1983-2009 at Coyote Creek Field Station, on the southern end of the bay. Each bird was caught, banded with an identification tag, and released after being weighed and measured either by the locally based PRBO Conservation Science or the San Francisco Bay Bird Observatory. Many were recaptured in later years, providing a data base that could track changes. Dozens of species were involved.
"The birds in the study included resident birds as well as migrants, so birds from Alaska or Central America were also showing in increase," Goodman said.
The birds grew on average by 2 percent in both mass and wingspan.
The mystery, however, involves a theory known to biologists as Bergmann's Rule, which generally holds that animals get bigger the farther north they are found. Ravens in Alaska are noticeably larger than ravens in the Lower 48. It applies to mammals as well: moose in the Arctic are much bigger than moose in Maine. The reason, the theory holds, is that larger animals can retain heat better and are therefore better able to survive at higher latitudes. Christian Bergmann, the 19th-century German biologist who came up with the rule, thought it applied only to warm-blooded creatures but scientists have found it applies to some cold-blooded animals as well.
So, as the climate warms, the birds should not be getting bigger.
The only other similar study of North American birds was done by University of Zurich's Josh Van Buskirk in western Pennsylvania. The birds he studied shrunk, obeying the rule.
Researchers from Cambridge University and the University of Tel Aviv, working in England, studied passerines (bullfinches and tits), and also found they were getting smaller just as expected.
"We expected them to show a decrease based on the first few studies," Goodman said of the California birds. "That left us with the puzzle of figuring out what was going on."
In the study, published in the journal Global Change Biology, Goodman and LeBuhn suggested two reasons for this exception.
One is that the birds are storing more fat to counter the greater number of extreme weather events that most climate scientists link to climate change. Bigger birds can survive storms better than can smaller ones so natural selection is choosing those with bigger bodies and wingspans.
The other theory is that the birds are eating a different diet. As the climate warms, plants change as does the population of insects birds might eat. That could contribute to larger birds.
Wesley Hochachka, senior research associate at the Cornell Laboratory of Ornithology, who studied the size of birds in Germany (they also shrank), had a slightly different interpretation. He said another explanation is that changes in temperature had nothing to do with it.
"It's not thermal at all," Hochachka said. Factors such as long droughts brought on by El Nino and the extent of the food supply are the most likely reasons. "Basically, Bergmann's rule is irrelevant."
Joel Shurkin is a freelance writer based in Baltimore. He is the author of nine books on science and the history of science, and has taught science journalism at Stanford University, UC Santa Cruz and the University of Alaska Fairbanks.