BOISE, Idaho - For decades, the sage grouse has struggled to compete against wildfire, development and the oil and gas drilling taking place across the West's rangeland.

Now, a wildlife biologist in southwestern Idaho is studying whether the chicken-size bird has another significant foe: West Nile virus.

Daniel Gossett, hired by the Shoshone-Paiute Tribes at Duck Valley Indian Reservation, said West Nile could have a devastating effect on the popular game bird's numbers.

Of all the threats sage grouse face, West Nile is the worst because "it has the quickest impact and is the one we can do the least about," said Gossett, whose two-year study is funded by a tribal wildlife grant program sponsored by the U.S. Fish and Wildlife Service.

Sage grouse once inhabited sagebrush and rangeland from Washington and eastern California across Idaho, Wyoming and Montana and east to North Dakota and Kansas. In the early 1800s, populations were pegged at 1.1 million, but in recent decades it has dwindled to fewer than 500,000, according to some state and federal estimates.

Environmentalists and state biologists say oil and gas drilling, ecosystem degradation and road building have destroyed or altered the bird's habitat. The recent Murphy Complex of fires in southern Idaho and northern Nevada, which burned for 18 days and charred more than 1,000 square miles, destroyed about 75 sage grouse mating sites, known as leks, according to federal wildlife managers.

West Nile virus, which appeared in the U.S. in 1999, was first reported in sage grouse in 2002. Last year, Idaho joined four other states in reporting the first sage grouse fatalities from the virus, the largest being a 70-bird die off in Oregon.

Gossett, a falconer who says he experienced the severity of West Nile a few years ago when the virus wiped out his entire raptor population, said the sage grouse is a species particularly susceptible to the mosquito- borne virus.

In most cases, Gossett said, the virus kills birds within three to six days and is fatal in 90 percent of infected birds. There is no treatment or vaccine for birds, he said.

At such potent levels, Gossett said a major outbreak could have a quick and devastating impact on the estimated 2,000 sage grouse living on the reservation.

As part of his study, Gossett spends his nights capturing the birds, using bright lights and music to entrance the birds before they are netted and fitted with a collar and radio transmitter. Idaho game biologists have also collared about 50 birds in southwestern Idaho as part of its West Nile monitoring program.

State officials say they need more data before voicing major concerns about West Nile on the state's sage grouse population.

Still, the state and the tribes have taken steps to close or restrict hunting in certain areas after the virus surfaced in birds. Hunting remains closed on the Duck Valley this year, and Gossett said he will recommend the tribes keep it closed until the disease is better understood.

Last fall, the Idaho Department of Fish and Game closed hunting in prime habitat in Owyhee County in the southwest corner of the state because of the virus, but no decision has yet been made on this coming season, officials said.