“Migratory birds are probably the least likely way avian flu is going to enter the Western hemisphere,” says Peter Marra, a bird ecologist at the Smithsonian Institution in Washington, D.C.

The more likely route into the USA, he says, is through the pet trade and the movement of poultry, legally or illegally. “Migratory birds are innocent bystanders,” Marra says. “I don’t doubt (they’re) moving the virus. I just don’t think they’re the primary movers.”