WASHINGTON, May 17 (UPI) -- U.S. researchers have determined the West Nile virus has apparently caused the continental-scale decline of seven species of birds across North America.

The finding by Shannon LaDeau and colleagues at the Smithsonian Institution in Washington is said to be a stark example of the potential havoc that can be wreaked by invasive species, and should impact future conservation strategies.

Researchers used 26 years of Breeding Bird Survey data to determine the impact of the West Nile virus on 20 North American potential bird hosts. They found declines in seven species from four families, including the American crow population, which has dropped by 45 percent since the virus emerged.

The declines occurred as the virus moved across North America and were significantly correlated with the intensity of transmission to humans. Only two of the seven species recovered to pre-virus levels by 2005, and that, the researchers said, is likely to have resulting effects for various ecosystems.

The study appears online in the journal Nature.