When Alfred Hitchcock released the 1963 horror film "The Birds," he probably didn't intend for his target audience to be the birds. But they may have been watching, waiting and learning.


At least, that's one fanciful theory to explain a recent study out of San Francisco State University which found that birds on both ends of San Francisco Bay have been mysteriously growing larger ever since Hitchcock made his iconic film.


The good news is that the birds probably aren't getting bigger because they've been secretly training for an imminent attack. The bad news, though, is that their increased size may be yet another ominous symptom of climate change, according to InsideScience.org.


The research, conducted by San Francisco State graduate student Rae Goodman and assistant professor Gretchen LeBuhn, involves data collected from 14,735 birds captured since 1971 near the northern tip of San Francisco Bay, and 18,052 birds captured between 1983-2009 on the bay's southern end. The birds chosen for the study represented over a dozen species. They were each tagged and monitored over time.


The findings showed that birds of all species had increased body mass and longer wingspans by an average of 2 percent.


"The birds in the study included resident birds as well as migrants, so birds from Alaska or Central America were also showing in increase," said Goodman.


Those results are particularly puzzling because the researchers had initially hypothesized that the birds would show a decrease in size, not an increase. That hypothesis was based on a biological theory called Bergmann's Rule, which says that animals tend to get bigger the further north they are found, since a greater bulk helps them to retain heat in colder climates. Because many previous studies had confirmed Bergmann's Rule, and since the planet's climate has been steadily warming, researchers thus assumed the birds would be getting smaller.


"We expected them to show a decrease based on the first few studies," said Goodman. "That left us with the puzzle of figuring out what was going on."


Goodman and LeBuhn still think climate change is the likely culprit, but in a different way than they first anticipated. They have proposed two possible explanations. First, the birds might be bulking up to better survive extreme weather events that have become more frequent due to climate change. Second, the birds might also be eating a different diet richer in nutrients, brought on by changes to plant and insect populations due to a warming climate.


Other researchers think there may be some other reason for the birds' mysterious growth altogether, perhaps in spite of climate change.


In response to the San Francisco study, Wesley Hochachka, senior research associate at the Cornell Laboratory of Ornithology, has suggested: "It's not thermal at all. 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."

Bryan Nelson ( @@brynelson ) writes about everything from environmental problems here on Earth to big questions in space.

California birds have been growing larger since release of famous Hitchcock film
Since Alfred Hitchcock made the 1963 film 'The Birds,' birds in San Francisco have been bulking up. Hopefully they weren't inspired.