CBS News reports on a study by The Audubon Society relased this week which “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 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.

Read the full story at CBS News. Photo by Audubon Society.

Here is the CBS News video.