All species of birds tested to date support replication of some HP H5N1 strains and, providing they are not killed rapidly, could contribute to the spread of H5N1 HPAI viruses. The present review has concentrated on ducks, some species of which are susceptible to H5N1 HPAI virus-caused disease and death while others (e.g., mallards) are quite resistant. 34, 50 Therefore, ducks that are infected but are naturally resistant to disease could have contributed to the spread of H5N1 HPAI viruses westward from Qinghai Lake in 2005 to Europe, Africa, India, and the Middle East. An unanswered question is whether the H5N1 HPAI viruses are being carried back to the duck breeding areas and are infecting the next generation. Extensive surveillance in the migratory pathways in Europe and Asia has provided no evidence of H5N1 HPAI viruses in the new generation of birds after the breeding season.