ANTIGO, Wis. - In an old incubator salvaged from a hospital nursery, a young red-tailed hawk lies flat on its belly, swaddled in a white terrycloth towel and a pink blanket.

Except for the rise and fall of its body as it breathes, the bird is utterly still. It doesn't react when Marge Gibson, a raptor rehabilitator, puts her hand inside the incubator.

"Her eyes are open, but we don't know how much she's taking in," Gibson says.

At least the hawk is alive. She is a survivor, for now, of a mysterious outbreak of neurological illness and death among hundreds - and quite likely thousands - of hawks, eagles, owls and falcons across the upper Midwest.

Since early August, from Iowa to Kentucky to Ontario, raptors have been found starving, anemic and dehydrated, unable to walk or fly, or swallow. Those that survive suffer from high fever, paralysis, tremors, weakness, convulsions and partial blindness.

"We've seen epidemics before," said Patrick Redig, director of the Raptor Center at the University of Minnesota, who has been working with birds of prey for almost 30 years. "But this eclipses anything I've ever seen, and certainly anything I've ever imagined."

The die-off may extend beyond the raptors. A recent Audubon Society report noted the scarcity of crows in the Chicago area. And Midwestern birders have noted the disappearance of many songbirds.

"I spend a lot of time canoeing on the St. Croix River," Redig said, "and it is just darn quiet out there."

A prime suspect is the West Nile virus. The mosquito-borne pathogen causes encephalitis, an inflammation of the brain, primarily among birds and horses, although more than 2,200 humans also have been stricken in the United States this year - 108 fatally.

Many of the raptors are testing positive for the virus. But if this is a West Nile epidemic, investigators say, it is unlike anything seen since the virus began moving south and west from New York in 1999.

'So darn surprising'

Until now, its arrival has always been signaled first by the wholesale deaths of crows, but only a handful of raptors.

"That's the thing that is so darn surprising to all of us," Redig says. "We knew it would get here sooner or later. But we didn't think it would hit raptors so hard. It begs the question: What changed? I don't know."

There is rising concern that as the virus spreads west, it will threaten the recovery of such rarities as the aplomado falcons in Texas and 73 California condors living wild in the Southwest.

At the nonprofit Raptor Education Group Inc. in Antigo, Gibson, her husband, one paid assistant and a shifting cast of volunteers are racing to save desperately sick birds.

"On Aug. 16, we got our first case," she says. It was the red-tailed hawk, found by the side of a road. The bird was presumed dead until Gibson's assistant noticed an eyelid flicker.

Then the phone calls started coming "in droves," Gibson said. There were seven to 10 a day. Birds were being brought in from all over Wisconsin and the adjoining Upper Peninsula of Michigan - Cooper's hawks, sharp-shinned hawks, bald eagles, great horned owls, red-tails, a merlin falcon and more.

"It was absolute chaos," she says. "We're not funded by the state or federal government. We operate on a small budget. We don't even have a secretary to answer the phone."

Each new arrival was added to a status board, in colored marker, with a brief description: a bald eagle, "toxic and very thin"; a red-tailed hawk, "dehydrated and starving"; a nighthawk, "didn't fly away"; another bald eagle, "partially blind."

There were 27 raptors on the board last week. Many, like the red-tail hawk, arrived virtually catatonic. Some were more responsive but disheveled, weak and feverish. Many others arrived dead, or died within a day or two.

The hawk in the incubator arrived with a fever too high to register on a thermometer calibrated to 111 degrees. "Normal" for a hawk is 104.

Many had to be hand-fed - rats and mice - or tube-fed with pureed meat baby food. "All of these cases are incredibly labor intensive," she said.

Gibson and her helpers soon began putting in 20-hour days. They canceled the center's educational programs. They canceled sleep.

Gibson, 57, a former medical technologist, has worked with birds for 40 years. She and her husband, Don, a retired pathologist, built the rehabilitation center 10 years ago. It stands on 70 acres of woods and meadow outside Antigo, about 200 miles northwest of Milwaukee.

With state and federal permits, Gibson cares for raptors, sandhill cranes, trumpeter swans and other birds found orphaned, poisoned, shot, burned by power lines, hit by cars or just sick.

'Hadn't seen it at all'

At first, she says, when all the raptors began arriving with neurological symptoms, "we were thinking organophosphates" -herbicide or pesticide poisoning.

West Nile had just begun to kill crows in Milwaukee last year. "But we hadn't seen it at all, not even crows," she says.

So when a West Nile test for the red-tailed hawk came back positive, Gibson says, "it was a real eye-opener." She immediately began hammering mosquito netting over all the "flights" and cages where recovering birds and those used in her educational work are kept.

Elsewhere across the Midwest and Canada, other rehabilitators were being similarly inundated.

Redig says a breeder in Ontario lost 69 captive owls to the illness. His center in Minneapolis has seen a tripling in the number of owls admitted for care, and twice as many Cooper's hawks and red-tails. In all, more than 60 raptors are recovering there.

Early last month, reports that great horned owls and red-tailed hawks were sick and dying in Ohio began reaching the National Wildlife Health Center in Madison, Wis., which investigates wildlife deaths for the U.S. Geological Survey.

Because the virus was already known to be active in Ohio, there was no great alarm. But within a week, calls began streaming in from Indiana, Michigan and Kentucky, and then from Iowa, Wisconsin and Illinois, and later from Minnesota, Virginia and Pennsylvania.

Some rehabilitators reported that they were getting 100 to 150 calls a week - 10 times the normal number. And these were just the birds that were being found.

"If it's just the tip of the iceberg, there could be real serious losses to those populations," says veterinarian Kathryn Converse, a wildlife disease specialist at the national center.

So, early this month, wildlife veterinarian Emi Kate Saito, who is coordinating an investigation into the deaths, asked rehabilitators and state health officials to send her a sampling of their freshest raptor carcasses for a full diagnostic work-up. FedEx trucks soon began dropping them off at the national center's back door, iced down in coolers.

In a basement biohazard lab, pathologists donned protective aprons and filtered breathing hoods. They checked the birds' brains, livers, kidneys and spleens for obvious abnormalities. Then they extracted tissues for microscopic examination and prepared samples to test for a broad range of bacteria, viruses and fungi, parasites, pesticides or toxic chemicals.

Scientific speculation

There is no shortage of scientific speculation. It's possible, Saito says, that the virus has killed off so many crows that infected mosquitoes have been forced to turn to raptors. Perhaps there are simply more infected mosquitoes, and raptors are more likely to be bitten and infected by one.

It's possible the virus has mutated, becoming more likely to cause illness and death. Perhaps raptors in the East and South were protected by exposure to another virus closely related to West Nile. Or perhaps another species of mosquito, or another insect entirely that prefers to feed on raptors, has picked up the virus.

There has also been speculation that raptors are not being infected by mosquitoes, but by exposure to the flesh and blood of infected prey. That would seem consistent with recent findings that the virus can be transmitted between humans in donated organs and blood.

Redig said the experimental work has not been done to prove it, but Gibson is convinced that mosquitoes are not vital to the cycle of West Nile infection.

She says a crippled bald eagle she uses as a foster parent to orphaned eagles became West Nile--positive after feeding infected younger birds. The large flight room where they were kept was netted against mosquitoes.

And when some of the 30 orphaned common terns she was caring for began dying, she sent their organs off for West Nile testing. All were negative at first.

But on Wednesday, her phone rang with word that the single tern brain sent to the lab has tested positive for West Nile.

"Damn!" Gibson says. "We gave the terns West Nile." The terns, endangered in Wisconsin, had been kept on a porch double-netted against mosquitoes. But Gibson had been hand-feeding them, and she's convinced, despite her infection control efforts, that she transmitted the virus from the ailing raptors to the previously healthy young terns.

"I wish we had all the answers," she says. "We just don't. It's scary."

While she waits for science to provide them, sick birds continue to turn up. Two more convulsing bald eagles arrived in Antigo on Thursday from northern Wisconsin.

But the news is not always gloomy. Returning to the examining room after checking on her other charges, Gibson finds the red-tail hawk standing in the incubator.

Six weeks after arriving nearly catatonic, the hawk eyes her suspiciously, following her movements as Gibson slowly opens the incubator lid. "This is brand new," she whispers.

Slowly and gently, she lifts the bird and holds it to her chest as if it were a human infant. "You're in there," she coos. "Somebody's home."