As climate shifts, birds follow

Most bird species in California’s Sierra Nevadas have altered their ranges during the last century in response to changes in temperature and rainfall

Climate has become warmer and wetter in parts of the Sierra Nevada mountains over the past century, and the vast majority of the birds there have shifted their range accordingly, a new study suggests.

ON THE MOVE The mountain bluebird (left) and Bullock’s oriole (right) are two of the dozens of bird species whose breeding ranges in California’s Sierra Nevadas have shifted over the past century due to climate change. M. Tingley

Over many generations, some plant and animal species can adapt to a slowly changing climate. When climate changes suddenly and dramatically, however, creatures generally shift their range, moving to new areas that offer suitable conditions, says Morgan Tingley, an ecologist at the University of California, Berkeley. Most previous studies have focused on species’ responses only to temperature changes. But the new study by Tingley and his colleagues — reported online and in an upcoming Proceedings of the National Academy of Sciences — shows that birds respond to changes in precipitation as well.

The researchers tallied all the species of perching birds present during breeding season at 82 sites along four transects, which stretch from low foothill elevations up to Sierra peaks topping out at more than 3,600 meters (12,000 feet). Then, they compared the results of their field studies, conducted from 2003 through 2008, with results of similar surveys at the same sites conducted from 1911 through 1929. In the intervening decades, the average temperature at the sites has risen by about 0.8 degrees Celsius and average annual precipitation has risen almost 6 millimeters, says Tingley.

In general, each species was present only in a rather narrow band of elevation — a sign that the creatures were adapted to a specific range of environmental conditions. Of the 53 species that were common at several sites both in the early 1900s and today, 90 percent had moved to a new breeding range during that time, says Tingley.

While most of those species had shifted to follow only one environmental factor — either increased temperature or increased rainfall — about 16 percent had shifted their range according to both. Birds that breed at low elevations tended to follow the rainfall, moving to wetter areas, while those that breed at higher mountain elevations tended to shift their ranges to follow the temperature most suitable for them, the researchers note. “That segregation in behavior surprised us,” says Tingley. The researchers speculate that this pattern may occur because some species, particularly those in lowland areas, are more constrained by food availability than they are by temperature.

“This is a landmark paper,” says A. Townsend Peterson, an evolutionary biologist at the University of Kansas in Lawrence. “It’s easy to note range changes for one species, but it’s quite different when most of the species are responding,” he says.

The five bird species that didn’t show any shift in range between surveys, such as Anna’s hummingbird and the western scrub jay, are species that have adapted to thrive around humans. Those species may be staying near areas where gardens and birdfeeders are common. The few species that shifted their breeding ranges in counterintuitive ways may be responding to other factors, such as extremes in temperature rather than averages, says Tingley.

Techniques used in the new analysis, as well as the findings, may help conservationists better estimate how species will respond to future climate change, Tingley says.

But because species didn’t respond to the climate changes in a consistent way, such ecological planning will be a challenge, says Peterson. “If everything moved in the same way, planning for conservation would be easy,” he notes.

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