But when a new virus gets into a barn packed with thousands of young chickens that have been genetically selected for their plump breasts rather than their ability to survive in the wild, it leaps from bird to bird, mutating slightly each time, and sometimes morphs into a lethal strain -- just as the 1918 Spanish flu was believed to become more deadly as it passed through crowded American military camps during the cold winter of 1917.
The new, lethal virus may then be passed back to migratory birds when they visit ponds or rice paddies used by domestic flocks -- or perhaps through intermediaries like crows or mynahs that steal food from poultry operations. But then, that wild infection may snuff itself out again because dying birds cannot fly far.
This may help account for the unpredictable hopscotch pattern the virus is following, experts said.
"It's hard to know what's going on," said Rob Fergus, science coordinator for the National Audubon Society. "But we're not seeing many wild birds dying nearby every time we find outbreaks in poultry."
Another possible route is the international trade in day-old chicks. After the outbreak in Nigeria, the Nigerian agriculture minister, Adamu Bello, said poultry producers there import chicks from China and Turkey, where the disease is widespread. Although the river deltas of southern Nigeria shelter many birds that migrate from Europe, ducks there are not dying, and the first infected poultry operations were in the arid north.
The chick trade "has made the chicken the most migratory bird in the world," said Adrian Long, a spokesman for Birdlife International. "When the United Nations puts every outbreak on wild birds, they're not exactly being Sherlock Holmes in their inquiries."
Two localized die-offs of wild birds, one in Azerbaijan and one in Germany, have been reported. And from Italy to Sweden, small numbers of dead, infected swans have been found, while many other species sharing waters with them appear healthy.
Many of the dead were mute swans (Cygnus olor), Poole said, which often live close to humans in park lakes. Unlike whistling swans, which migrate thousands of miles to the Arctic, mutes tend to circle Europe following food crops, the way Canada geese circle North America, and might have caught their infections from poultry in Turkey or Romania.
Human overreactions have proved more of a threat. In January 2004, the Thai government briefly laid plans to kill 70,000 open-billed storks in two nature reserves, fearing they were spreading the disease after 200 of them died. It canceled the plan a month later after only two of the dead birds turned out to have had the flu.
In Romania, villagers were reported to have attacked exhausted wild birds on an icy lake. In Maine, Kathleen Gensheimer, the state epidemi-ologist, said her department had fielded calls from citizens asking if they should shoot all the Canada geese landing in their backyards.
No bird with the deadly H5N1 strain has yet been found in North or South America.
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