BOISE, Idaho (AP) - 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-sized bird has another significant foe: West Nile virus.

Daniel Gossett, a wildlife biologist 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 right now, 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.

First identified by explorers Meriwether Lewis and William Clark, the sage grouse once inhabited the vast sagebrush and rangeland of the West, stretching from Washington and eastern California across Idaho, Wyoming and Montana and east to North Dakota and Kansas. In the early 1800s, sage grouse populations were pegged at 1.1 million across its historic range, 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.

Despite the bird's decline, the federal government denied a 2004 petition to list the sage grouse as an endangered species. Environmentalists responded by challenging that decision in federal courts, asking a judge to reverse a decision they argue was fraught with bad science and political interference by the Bush administration.

Last month, U.S. District Judge B. Lynn Winmill heard oral arguments in the case and a ruling is pending.

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 a county in eastern Oregon.