What's more, some birds stray off course and "can basically show up anywhere," Rosenberg added.
The first cases to reach North America, however, are likely to be isolated.
"A whole series of steps has to happen for wild birds to pose a problem," Rosenberg said.
To begin with, birds coming up from Asia or Europe have to be infected with the disease but healthy enough to fly. Second, any newly infected North American animals would have to survive long enough to carry it back south.
Furthermore, there are indications that the strain of the virus that kills poultry and humans isn't quite the same as the one infecting wild birds.
"I know of no documented cases of domestic birds picking up the disease from wild ones," Rosenberg said. Nor, he added, does he know of any documented cases of humans getting the disease from wild birds.
"At this point the virus is more of a danger to wild bird populations than wild bird populations are to humans," he said.
Even so, the disease is unlikely to cause a massive wave of extinctions, he said, although there are concerns about what would happen if the disease made its way into an endangered species.
The real concern is that bird flu might move from wild birds to domestic poultryand from there to people.
Allan Baker is head of the Department of Natural History at the Royal Ontario Museum in Toronto. He believes it is "almost inevitable" that the disease will ultimately reach the Americas, but he's not sure it will come via migratory birds.
The international trade in live poultry, he says, poses a far greater risk.
Nor, said Edward Dubovi, director of the virology section at Cornell University's Animal Health Diagnostic Center, should we discount the possibility that "stupidity and greed" might bring the virus in by an entirely different pathway.
Exotic bird smuggling is routine, he said, and pet-bird swap meets are essentially unregulated.
Rather than coming in via a wild bird, the disease may therefore arrive through "somebody who doesn't give a damn about anything but making a dollar."
Any infection of North America's domestic flocks could be an economic disaster. But large commercial flocks in the U.S. and Canada are fairly well protected, Dubovi said.
"If you go in one of those [poultry farms], your car gets washed down. You have to shower. They're probably not going to be all that vulnerable."
A bigger risk would be among organic farmers, free-range chicken farmers, and hobby farmers, whose flocks aren't as well barricaded.
Many of these people would report the disease and accept compensation for the slaughter of their flocks. But some might be reluctant to come forward, creating smoldering pockets of infection.
To the extent there is a risk to U.S. farmworkers, Dubovi believes it will also lie with the smaller operators: people who have 40 or 50 chickens and suddenly find 4 or 5 dead.
"We're accumulating lists of bird owners," Don Hansen, state veterinarian for the Oregon Department of Agriculture, told today's Oregonian newspaper.
"There are bunches of backyard folks," he added, including people in urban Portland, Oregon, who raise chickens as pets.
But Baker thinks that the risk of humans contracting bird flu in North America will be lower than in Asia.
That's because North Americans rarely live in close contact with their poultry as many Asian villagers do, he says.
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