Researchers in California have found that birds around San Francisco Bay and Point Reyes National Seashore have slowly gotten bigger over the last 27 to 40 years. The finding is certainly interesting—most of all, because it counters some conventional wisdom on how animals respond to climate change.
A well-known ecological rule, called Bergmann's Rule, states that animals tend to be larger at higher latitudes. It is believed that this occurs because larger animals are better able to conserve body heat. Bergmann's Rule, then, would suggest that animals will shrink in size as their native climates get warmer.
"We had the good fortune to find an unexpected result—a gem in research science," Rae Goodman, the researcher at the San Francisco State who led the study explained, "but we were then left with the puzzle of figuring out what was going on."
They came up with several possibilities, all indicating that birds—and ultimately other animals—may respond to climate change in unexpected ways. One theory is that the larger body size was caused by fat stores which have increased in response to increasingly common severe weather events. Another was that changes in plant ecology could have altered the birds' diet, leading to increased body sizes.
Regardless of the specific mechanism, it is clear that climate change is causing significant shifts in traits—like body size—that don't usually exhibit much flexibility.
Though the finding was unexpected, however, it was not necessarily all bad news. "It gave me a little more hope," Study Co-Author Gretchen LeBuhn explained, "that these birds are able to respond—hopefully in time—to changes in climate."