More bird species in the USA are ranging farther north and even staying there for the winter in a possible sign of adaptation to global warming, ornithologists and conservation groups say.

Some indicators come from the recent Great Backyard Bird Count, which found more swallows, orioles and other common birds in uncommon locations.

"We've got Baltimore orioles in 14 states, orchard orioles in five different reports and Scott's oriole in Pennsylvania. They shouldn't be here. They should be way south," says Paul Green of the National Audubon Society, co-sponsor of the count with the Cornell Lab of Ornithology.

Scientists cannot say yet whether the birds' movements are climate-related or a short-term reaction to storms, hot or cold spells, disruption of habitat or food availability.

However, the results of the four-day tally performed in February are "a tempting indicator of change, which may turn out to be the early stages of the effects of changing climate on bird distribution," Green says. "We won't know for certain until we have another 20 years of data."

Birds may have nature's best advantage against temperature rise: They can fly away. That can make them bellwethers of climate change, says Cornell ornithologist David Winkler. Birds "really are the canary in the coal mine - a very sensitive indicator of what's going on in the environment," he says. "We will see changes in their distribution long before we see big drops in their abundance."

Unusual sightings this winter include tree swallows in 22 states, twice as many as a decade ago, and red-bellied woodpeckers in New Brunswick, Canada.

Greg Butcher, Audubon's bird conservation director, says, "We know that things like Carolina wrens, eastern bluebirds, robins, crows and mourning doves are all spending winters farther north, and probably many more."

The backyard count, started in 1998, and an annual Christmas tally since 1900 provide two sets of data critical to gauging how birds react to warming: sheer numbers of birds and movement of species beyond traditional ranges.

Other factors can account for unusual sightings, such as the growing popularity of backyard bird feeders and loss of native habitat to development. "But when you see populations of birds" shifting, Butcher says, "you take notice."

The American Bird Conservancy noted last month that seven warbler species have shifted more than 65 miles north in a quarter-century.

It also said a Colorado study found robins arriving two weeks sooner at breeding sites still covered in snow, hindering access to food. "As a result," the conservancy says, "the 'early birds' may not get the worm."