WASHINGTON — When it comes to global warming, the canary in the coal mine isn't a canary at all. It's a purple finch.
As temperatures across the United States have risen, the purple finch has been spending its winters more than 400 miles farther north than it used to.
And it's not alone.
An Audubon Society study to be released Tuesday found that more than half of 305 birds 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 along the latitude of Milwaukee, Wis., instead of Springfield, Mo.
Bird ranges can expand and shift for many reasons, among them urban sprawl, deforestation and the supplemental diet provided by 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 5 degrees Fahrenheit. That warming was most pronounced in Northern states, which have already recorded an influx of more Southern species and could see some Northern species retreat into Canada as ranges shift.