Some Birds Highly Susceptible to West Nile Virus
West Nile Virus has decimated populations of some North American bird species, while sparing others. Scientists from Smithsonian Migratory Bird Center and Consortium for Conservation Medicine attribute population declines in seven types of birds to the deadly virus.
The Smithsonian scientists also found the disease hadn't reduced the numbers of 13 other species.
Since West Nile virus invaded North America in 1999, it has spread throughout most of the United States and southern Canada, as well as parts of Central and South America. The disease has shown up in over 300 bird and 30 mammal species.
Some regions in the United States have seen the number of American crows drop by 45 percent. Crows are the birds most vulnerable to West Nile virus, facing certain death within a week once infected. They are also among the most infectious of animals when bitten by a virus-carrying mosquito.
Between 1999 and 2006, the virus also reduced populations of American robins, eastern bluebirds, tufted titmouses and chickadees (blackcapped plus Carolina). Deaths from West Nile virus caused blue jay and house wren numbers to drop, but they recovered by 2005.
For some species, declines were localized. Significant mortality of common grackles occurred in Maryland, but not elsewhere. Chickadees mainly suffered in eastern regions, but not western states where the virus arrived more recently.
For any endangered species whose numbers are already low, the lethal virus could be devastating. Alarming declines in some greater sage-grouse populations were caused by the West Nile epidemic. So as not to take any chances, vaccinations were given to captive-bred California condors and whooping cranes. So far other endangered birds, including Kirkland warblers and Florida scrub-jays, have escaped harm.
The types of birds identified as those most afflicted by the virus tend to thrive alongside people in cities and suburbs. They also get targeted by mosquitoes from the Culex genus that transmit West Nile virus.
Some bird species show no population declines from the virus, although their numbers have dropped since 1980 for other reasons. These include Baltimore oriole, wood thrush and song sparrow.
A. Marm Kilpatrick, Shannon L. LaDeau and Peter P. Marra. 2007. Ecology of West Nile Virus Transmission and Its Impact on Birds in the Western Hemisphere. Auk. 124(4): 1121-1136.
Shannon L. LaDeau, A. Marm Kilpatrick and Peter P. Marra. 2007. West Nile virus emergence and large-scale declines of North American bird populations. Nature. 444: 710-713.