A study of birds, including gulls (above), purple finch and chickadees, has found that they have moved their base northwards over the last 40 years. [Agencies]

An Audubon Society study released yesterday found that more than half of 305 birds species in North America, a hodgepodge of species that includes robins, gulls, chickadees and owls, are spending the winter about 56 km 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 the upper Midwest instead of the Midwest.

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 US climbed by nearly 3 degrees Celsius. That warming was most pronounced in northern states, which have already recorded an influx of more southern species.

"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 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.

The study "shows a very, very large fraction of the wintering birds are shifting" northward, said Terry Root, a biologist at Stanford University.

The research is based on data collected during the Audubon Society's Christmas Bird Count in early winter. At that time of year, temperature is the primary driver for where birds go and whether they live or die. To survive the cold, birds need to eat enough during the day to have the energy needed to shiver throughout the night.

Milder winters mean the birds don't need to expend as much energy shivering, and can get by eating less food in the day.