Transmission of the H5N1 virus by wild birds was first suggested by the deaths of thousands of bar-headed geese in May and June at Lake Qinghai in western China. These deaths suggested that the virus was transported by migratory waterfowl and was so reported in the August 19 issue of Science, the journal of the American Association for the Advancement of Science. However, later reports indicated that Lake Qinghai is neither as remote nor as clean as originally thought and that poultry farms in the area, many of which drain into the lake, could have been the source of the virus that infected and killed the geese.
Data from the fall migration and the wintering ranges of waterfowl and shorebirds do not support the hypothesis that the virus is spread by migratory birds. If the hypothesis were true then as the geese, ducks, plovers, sandpipers and others moved west and south across Asia, eastern Europe and into Africa, there should have been outbreaks of avian flu along the migratory route. In particular there should have been occurrences of avian flu among birds and humans in the western Mediterranean and Nile delta. No such outbreaks have occurred. The failure to find such outbreaks argues against migratory birds as carriers of the virus. Reports of the disease have actually declined since the late summer despite the concentration of large numbers of waterfowl in their winter quarters. Looking back at the pattern of outbreaks in the late summer and fall of 2005, there is little evidence that it derives from the migratory routes of birds.
What we can conclude is that birds are susceptible to the virus, but that they get it from domestic poultry. If true, then the virus may have evolved to exploit the short lives of poultry, in which a virus must reproduce rapidly, and crowded conditions of most poultry farms, which makes transmission from one chicken or duck to another relatively easy. Again, if true, this suggests that the shipment of poultry from one locality to another is a greater threat than the migration of wild birds. The deaths of wild birds coupled with the failure of the disease to show up along migratory routes also suggests that while wild birds are susceptible, they may be ill-suited to transmission of the disease. Their failure as carriers may be due to their susceptibility, which may not give the virus time to multiply in sufficient quantity to be transmitted.