Letters from the West

West Nile harder on birds than thought

A downey woodpecker
A downey woodpecker

West Nile virus has caught Idahoans’ attention because, when passed on through mosquito bites, it makes people sick and because it has killed thousands of sage grouse.

But a new study led by a Colorado State University team shows that many more songbirds than originally thought are dying because of the disease that is spreading north as the climate warms.

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.

The study did not include sage grouse.

The research team analyzed 16 years of 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.

Researchers were especially 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,” CSU researcher T. Luke George said. “Our study suggests that their survival is reduced when they are exposed to West Nile virus in the wild.”

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. But for rare species, the presence of West Nile virus will make it harder for populations to recover.

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.

“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.”

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

Rocky Barker: 208-377-6484, @RockyBarker