In most cases so far, wild birds have appeared to contract the disease from poultry rather than vice-versa. Once infected, however, wild birds are capable of carrying the virus with them when they migrate, causing concern that they could infect poultry flocks elsewhere.

Infection by poultry can cause outbreaks in wild birds. In outbreaks among wild birds, such as those reported in Azerbaijan, Croatia, Greece, Turkey, and other countries in autumn 2005, the birds may have contracted the virus from infected poultry flocks in the Black Sea region, then dispersed to warmer regions, carrying the virus with them.

So far, however, there does not appear to be widespread transmission of the virus along migratory bird routes. Most migratory bird populations appear to be healthy, except in isolated areas where outbreaks have occurred.

One concern is that the high pathogenicity H5N1 virus has been detected in some apparently healthy wild ducks, indicating that some birds may carry the virus without obvious symptoms. However, tests of migratory birds show that high pathogenicity H5N1 is not common or widespread. Only 6 of more than 13,000 wild birds tested in China have tested positive. There have been no other cases in 100,000 tests of healthy wild birds elsewhere in the world.

In spring 2005, some 6,000 wild birds died in an outbreak at Qinghai Lake in northwestern China. Genetic analyses of viral strains in China show that this strain is similar to a strain found in healthy migratory ducks at Poyang Lake in southeastern China, indicating that migratory birds have the potential to carry the virus as they travel. However, the study also found a number of distinct regional viruses that show more localized transportation of poultry is still playing the primary role in the spread of the virus within China.

In several cases, the first outbreaks of avian flu in new places were linked to contamination from nearby poultry or poultry products rather than to wild birds. For example, new outbreaks in India, Nigeria, and Egypt occurred in poultry and have been linked to the trade in poultry products.

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