What are the most common bird species in and around your yard? Chances are they’re significantly less common today due to West Nile Virus. While many humans have become ill and nearly 1,000 have died -- making it the most deadly mosquito-borne disease in the US today -- West Nile is mainly transmitted between mosquitoes and birds.

The virus appears to have different impacts on different bird species. And there also appear to be regional differences in how hard West Nile is hitting a given species. For example, Common Grackles were hit very hard in Maryland when West Nile first showed up there. But in other areas their abundance seems to have remained stable.

Likewise, the population of Eastern Bluebirds continues to increase in Illinois, but is decreasing almost everywhere else. Among other birds that have been widely victimized are American Crow, Blue Jay, Black-capped and Carolina Chickadees, Tufted Titmouse, American Robin and House Wren.

The authors of a recent study in Nature magazine designed a predictive model to test whether West Nile had caused significant declines in a wide range of species from gulls to owls. Their predictions were based on prior laboratory experiments that attempted to measure which birds mosquitoes prefer to bite and their susceptibility to infection once bitten. These results were then compared with real-world population survey data from breeding bird surveys conducted by birders and the US Fish & Wildlife Service over the past 26 years. 21 different kinds of common, widespread birds were studied.

WNV is known to have arrived in the US in New York City in 1999. Its impact can be seen when a species’ population declines suddenly in an area after the time where the virus is known to have arrived in that area, in the absence of other reasons.

Here are some of the results:

* High WNV was predicted -- and found -- for American Crow, Blue Jay and Tufted Titmouse. A high impact was also predicted for Fish Crow but their populations do not appear to have declined as sharply.

* Moderate impact was predicted for Black-capped and Carolina Chickadees, House Wren, American Robin, Northern Cardinal, Song Sparrow and Common Grackle. The former species all declined as predicted, but populations of the latter three did not decline.

* Low impact was predicted for Mourning Dove, Downy Woodpecker, Wood Thrush, Gray Catbird, and Northern Mockingbird. As predicted, negative population trends associated with WNV were not found for these birds.

Several of the above species have declined by almost 50% across entire regions of the country, and only two of the hardest-hit species (Blue Jay and House Wren) have shown signs of recovery after experiencing intense epidemics in 2002 and 2003.

The impact of the virus on many other birds that were not part of the study is unknown. Obviously, however, there is cause for grave concern. For example, Greater Sage-Grouse, a bird already losing ground fast due to habitat loss, is apparently declining even more rapidly now.

The enormous extent of these population declines graphically illustrates how devastating introduced pathogens can be to organisms that have no immunity. Fortunately our birds have not been completely wiped out, as were all the native birds of Hawaii that lived below 4,000 feet elevation when whaling vessels brought avian malaria to the islands in the 19th century.

West Nile Virus, the “bird flu” and so many other diseases are part of the biological downside of global travel and trade. These pathogens have the potential to completely alter our wildlife communities. In the case of WNV, our common backyard birds might soon become relatively uncommon.

Meanwhile, globalization will almost certainly spread infectious diseases faster and faster. While humans worldwide fear these consequences, they can likewise only add to the onslaught of human-caused problems faced by birds and wildlife worldwide -- such as climate change, habitat loss and introduced predators like cats.

Much more research on how to slow the spread of the virus and reduce its impact is needed, not just for our North American birds but for all species in the Western Hemisphere. Perhaps we can learn enough to prevent the emergence of similar threats in the future.

Peace and good birding,
Scott Cronenweth
Eons birding mentor
www.naturalpathwalks.com