Efforts to save North America's tiny population of critically endangered California condors suffered what biologists called a minor setback last week when four 3-month-old chicks at an Idaho raptor center died of West Nile virus.

Their demise leaves just eight condor hatchlings at the World Center for Birds of Prey in Boise, where biologists have been breeding the federally protected scavengers since 1994.

These are the first condors to die from the mosquito-borne virus at the center, a project of The Peregrine Fund. West Nile showed up in Idaho in 2003 and emerged a month ahead of schedule this year, as spring rains left pools where mosquito larvae thrive, state officials said.

Bill Heinrich, the center's species restoration manager, said the deaths aren't a devastating blow to his $1.3 million annual condor breeding program, but they're still disappointing. C

onsequently, the facility in 2006 will send fewer than half the 20 birds it transported last year for release at sites including near Arizona's Grand Canyon.

"Until the young are 90 days old, they're too young to vaccinate," Heinrich said. "These birds were just about ready to be vaccinated when they caught the virus."

Chris Parish, the center's condor project director in Marble Canyon, Ariz., said wild condors reproduce at a rate of a single, 5-inch, 10-ounce egg, every other year.

In captivity, the Boise center's 19 breeding pairs — condors mate for life — can produce multiple eggs every year.

"Given they're so slow to reproduce naturally, we're still cranking out the birds," Parish said, adding that young of many wild raptors experience mortality rates of up to 50 percent.

Idaho public health officials say West Nile has been discovered in 67 pools of water, 11 humans, 16 horses and 24 birds, including wild hawks, so far this year.

"The birds and the horses are the two main sentinels we have to alert us that there's West Nile virus in the area," said Tom Shanahan, a spokesman for the state Department of Health and Welfare. "It's progressing into ecosystems throughout the state."

There's never been a known human death in Idaho from West Nile virus, whose symptoms include a fever or headache, or, rarely, serious illness and death, generally in older people with weak immune systems, according to the U.S. Centers for Disease Control and Prevention.

Condors have been listed as endangered since 1967, and the World Center for Birds of Prey is one of just four U.S. programs — two are in California, one in Oregon — breeding birds for release into the wild.

Condors, which have nearly 10-foot wingspans, once ranged from Mexico to Canada. By 1987, however, there were just 22 documented birds. The population was decimated by shooting, lead and pesticide poisoning, and egg collecting. That prompted biologists to capture the remaining wild condors for a last-ditch breeding program.

Today, California condors still number just 299, with 159 in captivity and 140 in the wild in Arizona, California and Mexico, according to the Arizona Game and Fish Department. Most Boise-produced condors are sent to northern Arizona, starting at about 6 months of age, for release at Vermillion Cliffs.

It'll be at least a decade before the birds recover enough to upgrade their status to threatened from endangered, said Kathy Sullivan, condor biologist with Arizona Game and Fish.

Still, the Idaho deaths won't affect the recovery program long-term, Sullivan said.

"We're taking steps to counteract this," said Peter Jenny, acting president of The Peregrine Fund. "Whenever you deal with animals — and particularly wild animals — you do have losses. It's always tragic."