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Study finds millions more birds killed by West Nile virus than previously thought

Researchers now believe more than 37 million red-eyed vireos have been killed nationwide by West Nile virus.
Kelly Colgan Azar via Flickr Creative Commons
Researchers now believe more than 37 million red-eyed vireos have been killed nationwide by West Nile virus.

Scientists, including a team from UCLA, have increased bird mortality estimates due to West Nile virus by the millions nationwide since the disease was introduced to the U.S. in the late 1990s.

A study released on Monday by UCLA and Colorado State University researchers in the "Proceedings of the National Academy of Sciences" looked at 49 species of birds and found nearly half saw significant decreases in survival estimates because of West Nile virus. Previously, a third of studied species had seen those decreases.

The findings show impacts also vary across species, with some rebounding quickly after initial introduction of the virus. The majority, however, have suffered prolonged population decreases for years.

“We were surprised at the percentage of bird species that we looked at that had negative effects and also the fact that over half of those had this persistent effect due to West Nile virus,” said Ryan Harrigan, a professor with UCLA’s Institute for the Environment and Sustainability.

The researchers believe one species, the red-eyed vireo, lost more than 37 million individuals, or 29 percent of its population in the United States.

While the study increases estimates of mortality compared with previous research, extensive comparisons may be difficult. That’s because researchers used a new method for making their estimates. Instead of relying on data from annual bird counts by citizen scientists as previous studies did, the researchers looked at the survival rates of birds that had been captured and banded.

“We weren’t seeing those birds with bands returning nearly as often after West Nile Virus hit, so we can attribute that to, wow, the only thing that’s really changed in the environment was the arrival of that disease,” Harrigan said.

Using data gathered from more than 500 banding stations around the country, Harrigan said regional declines in survival corresponded with the introduction of West Nile virus to the areas.

“We didn’t go out and test a single bird for West Nile Virus, but I’m fairly confident that we caught the effects of the disease, simply because we were tracking the disease as it moved across the country,” Harrigan said.

Harrigan said the study considered climate and land use changes in its analysis but found West Nile virus to be the most likely factor causing the declines.

“Certainly it could be something else, but let’s put it this way, it would be a brand new surprise to me,” he said.