DENVER - As bird flu is spread continent-to-continent by wild birds, the seasonal migration that is normally one of nature's wonders is becoming something scary.
Could bird flu reach North America through migrating birds? Biologists in Alaska and Canada are keeping an eye out and say it's possible by next year.
Scientists from several agencies have been monitoring large flocks in the northern part of this continent since last summer, collecting both live birds and thousands of samples from bird droppings. The results of those tests are pending, but so far scientists have not found the virus that is spreading across Asia.
Of course, the bigger fear is that bird flu will mutate into a flu that is both contagious and deadly to people and which would quickly spread around the globe through international travel. The current bird flu is not easily spread to people.
But scientists are studying the virus' transmission among birds as well. In the United States, a consortium of government agencies is seeking $5 million over the next three years to test birds along their migratory routes in the Lower 48 states beginning next spring.
"The patterns (of the virus) in Asia right now would not suggest that it would come over to North America this fall," said Christopher Brand, chief of field and lab research for the U.S. Geological Survey's National Wildlife Health Center in Madison, Wis.
Here's why: Bird flu was observed spreading from domestic poultry to wild birds in Asia last summer in northern breeding grounds in Siberia. Most of those birds now are migrating south - along their distinctive routes called flyways - to India and Bangladesh; others follow southwestern routes to the eastern Mediterranean and even Africa.
So far, bird flu has been detected in both wild and domestic birds as far east as the Danube Delta in Romania. The virus was reported in poultry in Turkey, Romania and Russia.
"There has been a shift in the susceptibility of wild fowl to H5N1," acknowledged David Nabarro, chief U.N. coordinator for avian and human influenza.
Brand says that if those birds maintain the virus over the winter, they would have the opportunity to bring it back to northern nesting grounds in Siberia next spring and summer.
While most Siberian flocks don't try to cross the Pacific to North America, some do cross the narrow Bering Strait to Alaska.
If those birds mingle with birds from Alaska, "there is the possibility the virus could be transmitted to waterfowl or shorebirds that make their way here next fall," Brand said.
While many severely infected birds usually die within a few days and are unable to fly very far, other hardier varieties could carry the disease.
Among the Arctic species under suspicion are hardier, long-distance fliers like eiders, gulls and geese. "It probably will be spread by one that isn't killed very easily by it," Brand said.
Many bird researchers say more dangerous transmission routes are the commercial poultry trade and the illegal trade in parrots and other rare birds for pets and collections. In both cases, birds are raised and transported in very cramped conditions.
The lone case of bird flu in Britain was a South American parrot that died while in quarantine with birds from Taiwan.
Conservationists argue that if migratory birds were the key factor in spreading the virus, outbreaks could also have been expected in the Philippines, Taiwan and Australia, which lie along regular migratory paths for Asian birds.
Much is still unknown about the H5N1 virus - one of the most lethal of many bird flu varieties - and how it spreads from domestic to wild birds and vice versa.
"If avian influenza has one predictable property, it is that it is not predictable," said Ohio State University biologist Richard Slemons. "It has made a fool of us more than once."