The birds carried low-pathogenic avian influenza - the less dangerous form of bird flu - but flu viruses can and do swap genes and mutate into more dangerous forms, the researchers said.

A major risk is that the birds would infect pigs, which are suspected "mixing vessels" for new strains of influenza - notably the ongoing pandemic of H1N1 swine flu, the researchers wrote in the Biomed Central journal BMC Infectious Diseases, available here

"What is significant about the work is that for a long time folks thought this was just an issue in shorebirds and ducks," Tom Smith of the University of California Los Angeles, who directed the study, said in a telephone interview.

"We haven't been doing surveillance on wild passerines. Most of the money for doing disease work in birds comes when one works on game species."

Using National Science Foundation and National Institutes of Health funds, Dr Smith and colleagues teamed up with David DeSante at the California-based Institute for Bird Populations to test songbirds for avian influenza viruses at the same time they were banding the birds for other research.

They tested more than 13,000 birds from 225 different species in 41 US states, and found low-pathogenic avian influenza in 22 species, notably fox sparrows, Cassin's finches, Swainson's thrush and Western tanagers.

New reservoir

Ducks are considered the main reservoir for avian influenza viruses. Even highly pathogenic influenza rarely sickens most species of duck, but it wipes out chickens and sometimes other poultry such as turkeys.

Low-pathogenic viruses rarely make birds sick, and Dr Smith said it was not clear what effect the infections had on the songbirds. "They were certainly flying around," he said.

The team has also found avian influenza viruses in songbirds in Africa - work that has not been published yet.

UCLA's Trevon Fuller said the team looked at satellite data and information from the US Department of Agriculture and found a big factor was farming.

"There is less habitat for wild birds," Mr Fuller said. "So the wild birds are being forced to congregate in smaller areas, and with higher density there is more opportunity for infected birds to shed virus," he said.

"In farms you have things like starlings and cowbirds and house sparrows all interacting. Those are areas where you would want to be more vigilant."

H5N1 bird flu has killed or forced the destruction of more than 300 million birds, according to the World Animal Health Organisation (OIE).

It rarely infects people but has been documented in 500 people since 2003 and has killed 296 of them.

Experts say the danger is that the virus will evolve just slightly into a form that people can easily catch and pass to one another, causing the transmission rate to soar and producing a pandemic in which millions of people could die.

H1N1 swine flu is the result of a so-called reassortment between pig and avian flu viruses. It has so far been about as deadly as seasonal influenza in terms of numbers but has killed far more children and young adults around the world than flu usually does.

Further Reading

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You can visit the Avian Flu page by clicking here.