In Florida, for example, more than 1,000 lesser scaup ducks have succumbed to the virus. In New Hampshire, about 50 Canadian geese died in a single event. In the Great Plains states, wildlife experts have seen mass die-offs in snow geese.

"In addition, there's a host of other species, including black vultures and bald eagles and some of the other scavenging species, that were likely infected by consuming the carcasses of those waterfowl," says Richards.

It remains to be seen how much of a toll this virus will take on American bird species.