West Nile Devastated U.S. Bird Species

Anne Minard
for National Geographic News
May 16, 2007
West Nile virus or a similar disease could wipe out many of the U.S.'s backyard birds, profoundly changing some of the country's most familiar wildlife and ecosystems.

That's the finding of a new analysis of 26 years of data from the national Breeding Bird Survey—data that reveal the dramatic effects of the 1999 arrival of West Nile virus in the U.S.

Lead author Shannon LaDeau of the Smithsonian Migratory Bird Center and her colleagues found that species that thrive near humans suffered extremely high death rates from the disease.

Up to 45 percent of crows died after the virus arrived, with robins, chickadees, and eastern bluebirds not far behind.

Some of these populations had been increasing before the virus hit, which is a good indication that West Nile caused the declines, the authors write.

The disease may not completely wipe out bird populations on its own, the scientists add, but it is an alarming addition to existing population threats such as climate change and habitat loss.

"They're our backyard species, and we haven't been watching them as much as we're watching the other species, because people consider them safe," LaDeau told National Geographic News.

The study appears in this week's issue of the journal Nature.

Mosquito, Bird Link

Since West Nile Virus began its mosquito-borne spread across the U.S., the Centers for Disease Control and Prevention has logged more than 12,000 human West Nile cases and 960 deaths.

Spikes in bird deaths among some species—including crows, house wrens, and eastern bluebirds—were linked with numbers of human cases, which peaked in 2002 and 2003, the study found.

That's because mosquitoes that carry West Nile fare best around people, where sources of stagnant water used for breeding—including sewers, old tires, and forgotten watering cans—abound, LaDeau said.

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.

Growing Problem

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.

Unanswered Questions

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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