"These are not the rare, vulnerable populations we think of as being at risk due to introduced species. These are our everyday, backyard country birds," said Shannon LaDeau, an ecologist at the bird center who led the study with [Peter Marra].

The trend "suggests that West Nile virus could potentially change the composition of bird communities across the entire continent," Carsten Rahbek, an ecologist at the University of Copenhagen, wrote in a commentary accompanying the research in today's online edition of the journal Nature.

"American crows are often considered a nuisance, but when the crows go, do we get more rats?" LaDeau asked. "What other scavengers come in, and what happens to the bird populations that are regulated in part by crows?"