Birds shifting toward north, study finds

WASHINGTON — An Audubon Society study to be released today found that more than half of 305 birds species in North America 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.

Third on the list was the marbled murrelet, which is found yearround in the coastal region of Washington state; they tend to move inland to breed. They are a protected species. Marbled murrelets, however, are among those that the Audubon Society said climate change was probably not the main reason.

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.

“This is as close as science at this scale gets to proof,” said Greg Butcher, the lead scientist on the study 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 provides compelling evidence for what many birders across the country have long recognized — that many birds are responding to climate change by shifting farther north.

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.