West Nile virus has prompted a long-term crash in the population of bluebirds, crows and other bird species that once dominated the suburban landscape, according to a new study that dashes hopes that the disease might cause only a temporary drop.

©Centers for Disease Control
Culex quinquefasciatus mosquito (Centers for Disease Control)

The study is the first national look at how West Nile has affected a wide range of species since the disease reached the U.S. in 1999. As expected, crows suffered the most, declining by up to 45 percent in some regions. Other studies have shown that the virus kills virtually all crows that contract it.

The virus hurt seven species in all, and of those, only two - the blue jay and house wren - had bounced back nationally by 2005. For some species such as the American robin, the number of birds nationwide did not decrease by much, but West Nile seemed to halt what had been an upward trend.

Of perhaps the most concern, the study confirms that West Nile is a problem likely to afflict birds and humans for years to come, said Michael Ward, a conservation biologist at the University of Illinois who was not involved in the new study.

"Everyone thought West Nile would be here a couple of years and then be gone," Ward said. "But that's not what we're seeing. It keeps popping up year after year."

Some species fared better in Illinois than in other parts of the country, the study's authors said. The paper, published online by the journal Nature on Wednesday, found little effect in Illinois on eastern bluebirds, which remain below expected levels nationwide.

Like birds, humans contract West Nile through mosquito bites. Birds serve as a disease go-between, intensifying the virus before mosquitoes spread it more easily to humans. More than 23,000 Americans have contracted the disease since 1999, and 962 have died, according to the Centers for Disease Control and Prevention.

Suburban America appears to offer a ready home for the virus. The explanation has remained elusive, though experts believe suburbia offers a convergence of what the virus needs: stagnant water where mosquitoes thrive, bird-rich yards and nature preserves, and a surplus of human hosts.

"It's a landscape where we do our best to attract backyard birds, and then it's very difficult to imagine anyone keeping an entire yard free from stagnant water," said lead study author Shannon LaDeau, a researcher at the Smithsonian Migratory Bird Center at Washington's National Zoo.

LaDeau said that at least on the East Coast and in the Midwest, disease-carrying mosquitoes seem to prefer man-made habitats - sewers, tires, or anywhere stagnant water can collect, even in bottle caps.

For whatever reason, the effect on birds has been far greater in Chicago's near north suburbs than on the city's North Side, said Michael Ward, who has studied West Nile's effects in Illinois.

The difference in chickadee numbers can be audible at the city's borders, said Judy Pollock, director of bird conservation for the National Audubon Society's Chicago region office.

"The chickadee [difference] seems to be a very localized thing," Pollock said. "I've noticed for a while that once you cross Peterson Avenue going south, you can hear the chickadees again."

Charlotte Adelman, an amateur naturalist and author from Wilmette, said that a few years ago she thought West Nile had made chickadees "locally extinct" in the northern suburbs.

"In the last couple of years they've seemed to be making a comeback," said Adelman, who observes birds at her backyard birdhouses and feeders.

Although numbers of robins dropped off sharply in parts of the Eastern U.S., the numbers were flat in Illinois, according to the new study. In fact Ward said his data, which draws from a greater number of Illinois sources than the study in Nature, suggests robins actually have increased here.

That may be because West Nile kills off some older robins, leaving more space and food for younger birds that might never have survived otherwise, Ward said. He has seen a similar pattern among cardinals, which West Nile has not hit hard.

"These birds are so fertile and productive they can compensate" for small declines from the virus, Ward said.

Robins and cardinals also seem better able to develop antibodies that give them natural immunity to the virus, Ward said. In contrast, virtually no crows or chickadees are ever found with antibodies for West Nile, suggesting those birds cannot mount a defense against the disease.

None of the birds in the new study is truly endangered or at risk of going extinct, LaDeau said. But she said the virus has the potential to weaken some bird populations in unpredictable ways.

"If the population is already stressed, that may make it more vulnerable to drought, a particularly bad West Nile season, further habitat destruction, or the next pathogen that comes along," LaDeau said.

More important, the statistical difficulty of studying truly rare or endangered birds means there is little way to know what effect West Nile has had on those species, said Douglas Stotz, a conservation ecologist at the Field Museum.

"You probably couldn't get rid of crows if you tried, but some related species are endangered, and it's hard to tell what's happening with them," Stotz said.

For example, he said the yellow-billed magpie, which has a small range in California, could be vulnerable to an intense West Nile outbreak. The threatened Florida scrub jay, which is related to the common blue jay, also could be hit hard.

No one knows the long-term effects on the food chain of having fewer crows around. The birds normally are scavengers that also eat smaller birds, eggs, insects and mammals. Experts said a shortage of crows could produce an increase in other scavengers or the small animals that crows would otherwise eat.

Adelman said she believes crows previously helped keep rabbit numbers in check in her neighborhood by eating baby rabbits. Now that crows are scarce, she has noticed a local rabbit population explosion.

"I used to be very excited when I saw a rabbit," Adelman said. "Now they're just ubiquitous."