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 the temperature across the U.S. has gotten warmer, 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.