West Nile Virus and the birds

The following article was written by Laura Gooch, a former environmental engineer, now nature photographer and author of The Doan Brook Handbook. The article appeared in "Rookery," the monthly newsletter of the Nature Center at Shaker Lakes.

West Nile Virus (WNV) is in the news, and Cuyahoga County is a virus hotspot. We hear most about the virus's impact on humans, but WNV is primarily an avian disease. How does the impact of WNV on birds compare to its impact on people?

WNV sent 219 Cuyahoga County residents to the doctor or the hospital last year, and 13 of these people died. A study that drew blood samples from healthy residents led the Cuyahoga County Board of Health to estimate that between 52,000 and 83,000 people in the county were infected during 2002. That means that about 1 in every 20 people in our area encountered an infected mosquito and developed immunity to WNV. About 1 in 50 area mosquitoes was infected by the end of the summer.

What does all of this mean for the birds? The details of WNV's impact are difficult to state with certainty. Authorities believe that WNV has killed at least hundreds of thousands of birds, and some estimates of bird mortality are as high as tens of millions. Mortality rates among infected bird species vary widely. Between 95 and 100% of infected American crows die from the disease under laboratory conditions. Other birds have slightly or much higher survival rates.

Preliminary data from Cornell Lab of Ornithology's Project Feeder Watch indicate that some bird species in the upper Midwest declined over the summer of 2002. A look at last years' Cleveland Christmas Bird Count data for a few species indicates that some local populations may have mirrored these declines. Compared to the average populations observed since 1970, American crows declined by 81%, black-capped chickadees by 33%, and tufted titmice by 31% (based on birds seen per observer hour). Data for many other species haven't been examined.

While the declines in crows, chickadees, and titmice seem alarmingly large, it is hard to say what they mean, and even hard to attribute them to WNV with certainty. Many dead birds that were tested last year were infected with WNV, but there is no way of verifying that the virus caused their deaths. In addition, bird populations fluctuate from one year to the next for a variety of reasons. While the Cleveland Christmas Bird Count numbers for crows, chickadees, and titmice were among the lowest since 1970, only the American crow count was the lowest observed.

This much can be said with certainty: WNV infects many species of birds as well as humans, and it is far more deadly to birds than it is to us. The long term impact on some species of birds, and on the songbird population in general, could be significant. We won't know what the full impact is for a number of years.

What can you do to protect the birds? The same measures that are most effective in protecting people will also protect birds: eliminate standing water in your yard, fill water-holding tree cavities with sand or gravel, keep pools and hot tubs clean and chlorinated, clean birdbaths at least once a week. Remember that mosquitoes can breed in tiny amounts of water. Finally, encourage your city officials to take preventive measures like eliminating mosquito breeding areas, educating the public, and treating catch basins with larvacide. Aerial spraying is not a good way to protect birds, since it may be directly harmful to them and can kill insects that are part of their diet.

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