Forgotten but not gone: West Nile virus impacts greater number of bird species, study finds

Research funding for the West Nile virus has mostly dried up. But a new study led by Colorado State University finds that the deadly disease is killing birds — more so than previously thought — in the short- and long-term. The study findings have implications for the world’s ecosystem, human health, bird populations, and activities like bird-watching, which has become a thriving industry.

A team led by CSU researcher T. Luke George analyzed 16 years of mark-recapture data collected at 574 bird-banding stations across the United States from 1992 to 2007. They found large-scale declines in survivorship and a larger proportion of species — at a minimum, 47 percent — that may have been affected by West Nile virus.

CSU researcher Luke George with Jeff Birek at the Bird Conservancy of the Rockies
CSU scientist Luke George (right) with Jeff Birek, Bird Conservancy of the Rockies.


“Many more species of birds than we thought are susceptible to this virus,” said George, a senior research associate in the Department of Fish, Wildlife and Conservation Biology in the Warner College of Natural Resources at CSU. “We also found long-term effects on population growth rates of some species. Prior to this study, we generally thought the West Nile virus had a very short-term effect on bird survival.”

A bird is captured in a net in the bird-banding process.
Birds are captured in large nets as part of the bird-banding process.

The bird-banding stations are operated as part of the Monitoring Avian Productivity and Survivorship program, a cooperative effort among public agencies, private organizations and individual bird banders that was established by The Institute for Bird Populations, a California-based nonprofit that uses the data to examine the underlying causes of changes in bird populations.

Tracking the virus

The team, which includes researchers from the University of California Los Angeles, Washington University and The Institute for Bird Populations, first used a map of West Nile virus prevalence among humans to get a handle on the intensity of West Nile virus across the continental United States. They had a hunch that it might correlate to the effect on bird populations. Mosquitos move the virus from birds to humans.

George said researchers also tested another hypothesis: Is it possible that the virus has a delayed effect on birds? “It’s kind of like the flu,” he said. “One week, you detect the flu in schools, and the next week, 30 percent of kids in the school are absent.”

They soon saw what George describes as a “big deal” – an impact on a higher percentage of species than people had detected before, and that, for some species, the impact had long-term effects.

Birds are fitted with uniquely-numbered aluminum leg bands.
Birds are fitted with uniquely-numbered aluminum leg bands. Photo courtesy of the Institute for Bird Populations.

“For about half of the species that were affected, the impact is not just during the year when West Nile virus arrived, but we saw its influence for many years afterwards,” he said. “And we found this occurring all over the United States.”

Prior to this study, scientists did not really understand the extent of the burden of the virus. George said that several West Nile virus studies looked at specific locations, but the studies were short-term (two to three years) and looked at one species and one location. “Some people found effects, and others didn’t,” he said.

Some birds survive, others do not

The field sparrow, downy woodpecker and red-eyed vireo experienced significant declines in survival associated with the arrival of the virus, followed by recoveries. For others, such as the

Swainson’s thrush, purple finch and tufted titmouse, survival declined upon arrival of the West Nile virus and consistently remained lower.

George said he was surprised by the findings for the Swainson’s thrush, a species known for its flutelike songs. “Lab studies found that while Swainson’s thrushes were susceptible to West Nile infections, they always survived,” he said. “Our study suggests that their survival is reduced when they are exposed to West Nile virus in the wild.”

What are the implications for this research? “Birds are part of our national heritage,” said George, who also has a research affiliation with The Institute for Bird Populations.

“Birds feed on insects, disperse the seeds of many shrubs and trees, and are important because they pollinate many flowering plants that we enjoy,” he said. “Bird-watching is one of the biggest outdoor activities and what we’ve found means there are reduced opportunities to see the birds around us.”

The study suggests that West Nile virus is an additional factor that could reduce the growth rate of the bird population over the long-term. Since there are millions of thrushes, there is not an immediate threat to that species, George said. “For rare species, we’re adding another burden to their population. It may not lead to the extinction of the species, but the presence of West Nile virus will make it harder for populations to recover.”

The study, “Persistent impacts of West Nile virus on North American bird populations,” is published in the Proceedings of the National Academy of Sciences.

Co-authors on the study include Ryan Harrigan (UCLA), Joseph La Manna (Washington University), David DeSante and James Saracco (The Institute for Bird Populations) and Thomas B. Smith (UCLA).