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Bay Area hummingbirds and other small birds have been growing larger in recent decades

Scientists have discovered an unexpected consequence of climate change in the Bay Area: bigger songbirds.

The researchers found that the weight and wingspan of thousands of small birds — including finches, robins, swallows and hummingbirds — increased a small amount almost every year during the past three decades, according to a study published this month in the journal Global Change Biology.

The discovery contrasts with findings from another region in the country, highlighting the haphazard and sometimes unpredictable consequences of global warming.

“We were very much surprised,” said Point Reyes Bird Observatory research director Nat Seavy, one of the scientists who analyzed 40 years of bird measurements from Point Reyes National Seashore and 27 years of data from Milpitas.

The research concluded that the 73 bird species studied increased in size by 0.02 percent to 0.1 percent annually, according to Seavy. The scientists detected the growth in both migratory birds and birds that live in California their whole lives.

A similar study in Pennsylvania found that songbirds there are decreasing in size annually as the climate changes.

“One of the things that makes climate change difficult is that the way it changes ecosystems is going to be very different throughout the world,” Seavy said. “It’s going to manifest itself in different ways.”

Ecologists generally expect animals to become thinner and smaller as temperatures rise. According to Bergmann's rule, a theory developed in the 19th century, warm-blooded animals in cold climates will be larger than their cousins in warmer zones, because bigger animals are better at retaining body heat.

Seavy and his team expected the birds would decrease in size in order to stay cool as temperatures around the globe increased. But the counterintuitive findings suggest that global warming is affecting animals' body size in a different way.

While climate change brings warmer overall temperatures, it also increases the frequency of storms, droughts, wildfires and other conditions that sometimes force animals to endure extended food shortages.

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Climate change is causing Audubon's Warblers and other songbirds in the Bay Area to increase in size

The scientists who conducted the California study think birds could be evolving to be bigger in order to increase fat and muscle reserves to help them survive during those difficult periods.

“Organisms, and birds in particular, need to have some reserves to hold themselves through bad times,” said San Francisco State University ecology Professor Gretchen LeBuhn. “The frequency of bad patches is predicted to increase — and seems to be increasing.”

An alternative theory suggests that the birds are growing because lush, wet conditions associated with climate change on the West Coast have increased the plants, insects and other food available to them.

“It’s getting warmer and wetter here,” said Jill Demers, executive director of the San Francisco Bay Bird Observatory, which was also involved with the study.