One report concluded it could become a global threat.
If birds carry the H5N1 flu virus beyond its current stronghold in southeast Asia, it could devastate poultry farms and raise the risk of a deadly flu pandemic in people, experts said.
"They're going to spread this ... thing further and further across central Asia and Europe and who knows where," said Robert G Webster of the St Jude Children's Research Hospital in Memphis, Tennessee, an author of a report released on Wednesday by the journal Nature.
Another report, released by the journal Science, said the finding of the H5N1 infection in migrant birds at Qinghai Lake in western China "indicates that this virus has the potential to be a global threat".
The reports echo concerns voiced last week by the World Health Organisation, which urged China to step up its testing of wild geese and gulls. A WHO official estimated that the flu had killed more than 5 000 wild birds in western China.
The outbreak was first detected about two months ago in bar-headed geese at China's remote saltwater lake, which is a key breeding location for migratory birds that overwinter in southeast Asia, Tibet and India.
The virus has hit that species the hardest, but also affects brown-headed gulls and great black-headed gulls.
The H5N1 virus has been entrenched in poultry in southeast Asia since 2003, and variants of it infect people. Webster said the Qinghai Lake virus is genetically different from the one that has been infecting people in Vietnam, but it is a "first cousin ... not far away at all".
That implies it has the potential for infecting people, probably by way of domestic chickens or ducks, he said.
If a bird flu virus infects a person who also carries a human flu virus, the result could be a hybrid bug that passes easily from person to person.
"That's the spark that sets off ... a global pandemic, and that's what everyone is worried about," said flu expert Dr William Schaffner of Vanderbilt University.