From a remote corner of the Tibetan plateau in western China, park ranger He Yubang finds himself thrust onto the front lines of the global battle against avian influenza.

More than 5,000 migratory birds on his nature preserve in Qinghai province have dropped dead this year in a highly unusual outbreak of bird flu that has baffled scientists.

The World Health Organization sent a team to investigate in late June, and it warned that the tens of thousands of wild birds that use the preserve as a breeding ground during the summer would begin to migrate in September, potentially spreading the disease around the globe. To track the birds when they leave, the WHO asked the Chinese government to begin tagging them. International scientists and Chinese park rangers have only a vague sense of where the birds go and what routes they fly to get there.

Mr. He says he is trying to comply with the WHO's request, but he and his team haven't been able to catch a single bird. He has even stayed up until 5 a.m., lurking on the shores of the lake where the birds live, hoping to sneak up on them while they sleep. Whenever he manages to get within a few dozen feet, though, a bird acting as a sentry alerts the others and they all fly away.

"It seems we have to wait to do it until next year" when hatchlings are unable to fly, says Mr. He, deputy chief of Qinghai Lake National Conservation Bureau.

But scientists warn that waiting would squander a critical opportunity to learn more about the way the virus spreads. Though experts can't be sure migratory birds are responsible for spreading the disease all over Asia, they are prime suspects, mostly because migratory ducks and possibly other species have long been suspected of harboring the virus without dying, and they often mingle with domestic flocks along their migratory routes and may transmit the virus then.

More than 50 people in Thailand, Cambodia, Vietnam and Indonesia have died from contact with infected poultry since late 2003. Hans Wagner, senior animal production and health officer in the Bangkok office of the United Nations' Food and Agriculture Organization, considers migratory birds one of the principal ways the disease is spread.

The role of migratory birds has come under renewed scrutiny since Russian officials found bird flu near the Chinese border. Late last month, Russian health officials confirmed that the deadly H5N1 strain of the virus had cropped up in the Siberian region of Novosibirsk, killing some 1,300 farm birds in a region about 2,000 miles from China's Qinghai Lake. Four other regions in Russia also have had outbreaks of the virus, although it isn't clear which strain was involved. Gennady Onishchenko, Russia's top epidemiologist, blamed migratory birds for the arrival of avian flu in the country, and now other Russian health officials are warning that migratory birds could spread the disease further west.

In one affected part of Siberia, authorities say they will shoot wildfowl to prevent the disease's spread. Authorities said 4,061 wild and domestic fowl have died of the disease in Russia. Last week the European Commission, the European Union's executive arm, said it was imposing a ban on imported birds from Russia and Kazakhstan.

In addition, a team of scientists writing recently in the journal Nature warned that migratory birds in Qinghai have the potential to spread the disease well beyond Asia.

The U.S. Agriculture Department, wary of the threat posed by birds migrating from Asia, has set up an early-warning system in Alaska, testing ducks and geese that cross the Bering Sea from Asia for bird flu.

Disease experts worry that many of the birds still alive at Qinghai Lake may be harboring the virus but be asymptomatic, allowing them to continue with their annual migration. The worst-case scenario is that these birds -- which can travel thousands of miles, from Scandinavia to Senegal, for instance -- will become the epidemiologic equivalent of intercontinental ballistic missiles, capable of eluding even the strictest quarantine measures on the ground.

Still, other experts question how central a role migratory birds play in the global spread of disease, and some worry that rare species may be slaughtered unnecessarily. They point to the ground transport of domestic poultry as the most likely means of spreading the virus.

Nonetheless, many experts agree that more needs to be learned about which migratory species have a natural immunity to H5N1 and where they fly. "We need to track these birds," says Jeffrey Gilbert, a WHO infectious-disease specialist who traveled to Qinghai to see firsthand how local officials were handling the outbreak.

Birds have been blamed for spreading other deadly diseases, including West Nile virus, Japanese encephalitis, Nipah virus, salmonella and Lyme disease. In 1999, for instance, illness in crows in New York and then bald eagles at the Bronx Zoo presaged a human outbreak of West Nile virus that infected 60. Birds -- including crows, jays and magpies -- were suspected of spreading that mosquito-borne illness.

David Swayne, director of the U.S. Agriculture Department's Southeast Poultry Research Lab, which has been working with scientists at the University of Alaska in Fairbanks, says that so far none of the some 12,000 samples they have collected since 1998 have tested positive for the H5N1 strain of the virus, although low-pathogenic strains of bird flu have been detected.

But even as governments spend millions of dollars on preventive measures, tracking the birds remains a problem. Back in 1983, when Chinese park rangers tagged migratory birds that visited Qinghai Lake for a research project, they used nets, Mr. He says. The tactic worked, but too many birds choked to death as a result. More than two decades later, without nets, the task has proved far more difficult.

Every year tens of thousands of birds -- mostly bar-headed geese, cormorants and brown-headed gulls -- flock to one of the five islands in the middle of Qinghai Lake at the heart of the nature preserve. By August, the island is covered with bird eggs.

The State Forestry Bureau has ordered the team of about 20 rangers to tag some of the birds and has supplied numbered tags to attach to the birds' feet, according to Chu Guozhong, director of the bureau's National Bird Tagging Center. The tagging approach is being used because, at $1.23 apiece, the tags are relatively cheap. But even if the birds could be easily caught and tagged, Chinese officials would have to rely on bird watchers or agriculture officials in other countries to spot the tags and report the serial numbers back to a central office.

Electronic tracking devices can be implanted into a bird's back, but they cost thousands of dollars a bird, including the price of the receiving station necessary to monitor the birds' movements. The forestry bureau has asked China's Finance Ministry for more funding, and that request is pending.

"If it gets approved," Mr. Chu says, "we'll surely find a way to catch the birds."