From the The Lancet Infectious Diseases
Since mid-2005, the Food and Agriculture Organisation (FAO) and WHO have given wide prominence to the theory that migratory birds are carrying the H5N1 virus and infecting poultry flocks in areas that lie along their migratory route. Indeed, this is probably how the virus reached Europe. Unusually cold weather in the wetlands near the Black Sea, where the disease is now entrenched, drove migrating birds, notably swans, much further west than usual. But despite extensive testing of wild birds for the disease, scientists have only rarely identified live birds carrying bird flu in a highly pathogenic form, suggesting these birds are not efficient vectors of the virus. Furthermore, the geographic spread of the disease does not correlate with migratory routes and seasons. The pattern of outbreaks follows major road and rail routes, not flyways.
Far more likely to be perpetuating the spread of the virus is the movement of poultry, poultry products, or infected material from poultry farms—eg, animal feed and manure. But this mode of transmission has been down-played by international agencies, who admit that migratory birds are an easy target since nobody is to blame. However, GRAIN, an international, non-governmental organisation that promotes the sustainable management and use of agricultural biodiversity, recently launched a critical report titled Fowl play: the poultry industry's central role in the bird flu crisis.