In 1999, West Nile virus snuck into the United States. Like many border-crossing diseases, the virus wreaked more havoc in its new home, where those it infected hadn't developed immunity. While the virus infects humans, and can be fatal, it was deadliest in a different population: North American birds.
When the virus first appeared, Luke George, a researcher at Colorado State University who studies birds, said there were reports of dead birds. Lots of them.
"That was how people knew it had showed up in North America is they were seeing lots of dead crows and jays, especially in suburban areas."
George and other scientists knew West Nile killed other birds too, but many thought after an initial die-off most populations recovered. No one really knew the true effect on North American birds, until George and some other researchers decided to take a closer look.