Scientists Think Global Warming Leads Many Birds to Shorten Migration Trips

Wednesday, March 18, 2009

About 100 years ago, coal miners would take a bright yellow canary down into the mines with them. The miners wanted to hear the bird's sweet song, but not for the reason you might think. As long as the canary sang, the miners knew the air in the mine was clean and safe to breathe. If the canary got sick, they knew it was time to evacuate.

The expression "a canary in a coal mine" came to mean a warning device. When it comes to global warming, the canary isn't a canary at all. It's a purple finch.

As the temperature across the United States has gotten warmer, the purple finch has been spending its winters more than 400 miles farther north than before. And it's not alone.

A National Audubon Society study released last month found that more than half of 305 bird species in North America, a hodgepodge that includes robins, gulls, chickadees and owls, are spending the winter about 35 miles farther north than they did 40 years ago.

The purple finch was the biggest northward mover. Its wintering grounds are now more than 400 miles farther north.

Birds move for many reasons: They get chased away by more buildings and fewer trees, or they go in search of neighborhoods with backyard feeders. But researchers say the only explanation for why so many birds over such a broad area are wintering in more-northern locales is global warming.

Over the 40 years covered by the study, the average January temperature in the United States climbed by about five degrees. That warming was most noticeable in northern states, which have already recorded more southern birds.

"This is as close as science at this scale gets to proof," said Greg Butcher, the study's lead scientist and the director of bird conservation at the Audubon Society. "It is not what each of these individual birds did. It is the wide diversity of birds that suggests it has something to do with temperature, rather than ecology."

The study of migration habits from 1966 through 2005 found about one-fourth of the species have moved farther south. But the number moving northward -- 177 species -- is twice that.

The study "shows a very, very large fraction of the wintering birds are shifting" northward, said Terry Root, a biologist at Stanford University. "We don't know for a fact that it is warming. But . . . we know it is not just a figment of our imagination."

Changes in temperature affect different birds in different ways.

Some birds will expand their range farther north. For example, the Carolina wren has turned into a Yankee, based on Audubon's calculations. It is now commonly seen in the winter well into New England as well as its namesake state of South Carolina.

Other species, such as the purple finch, spend their summers in the forests of Canada and fly south into the United States for the winter. Climate change could be playing a role in why they are not flying as far south as they used to.

-- Associated Press

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