Likewise, birds found to suffer the greatest West Nile death rates also do well around people. For crows, "the more dumps, the merrier," LaDeau pointed out.
Not all of the 20 bird species studied in the paper showed the same response to the introduction of West Nile virus, though.
Crows, robins, chickadees, and bluebirds suffered steep, sometimes progressive, multi-year declines after the disease arrived. (Related: "West Nile Mosquitoes Prefer Robins, Study Finds" [September 25, 2006].)
Blue jays, tufted titmice, and house wrens, however, showed strong one- or two-year declines after intense West Nile virus epidemics, but little or no impacts at other times. Blue jays and house wrens had rebounded by 2005, in fact.
Other species seemed to do just fine in the face of West Nile. But that could be because the effects of the disease got lost in population fluctuations or long-term declines, the authors write.
The study results raise new concerns about bird species that aren't included in backyard bird counts, LaDeau added.
"We can't talk about impacts [of West Nile virus] to those species because we don't have the data," she said.
West Nile may be worst for bird populations when the disease is paired with other threats, the study authors point out.
In a May 12 press release commemorating International Migratory Bird Day, the American Bird Conservancy warned that migratory birds are still dying in large numbers from collisions with lighted buildings and communication towers, pesticide poisoning, and free-roaming cat predation.
New concerns, the organization said, include poorly placed wind farms and the spread of corn farming for biofuels, which may usurp vital bird habitats.
The conservancy organization estimates that more than a third of the 650 bird species that breed in the U.S. now have declining populations, are restricted to small ranges, or face serious threats.
Leslie Dierauf is director of the National Wildlife Health Center in Madison, Wisconsin.
She said the new Nature article is a good start because it's the first thorough look at the effects of West Nile virus on available bird numbers.
But it doesn't go far enough.
"I don't think they've brought enough of the complexities into the paper," she said.
It's quite difficult to tease out all the interrelated factors that can impact bird populations, such as habitat, climate, and diet, Dierauf added.
The next steps include comparisons of how closely related bird species to the ones studied fared in the face of West Nile, along with other surveys, she said.
Only then can experts make definitive conclusions about the West Nile's effects on U.S. birds.
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