While all duck species tested to date are susceptible to lethal infection with H5N1 HPAI viruses, some species, including the mallard and Pintail ducks, are less susceptible and many survive. It is in these relatively resistant species that the H5N1 HPAI viruses could be maintained. However, surveillance studies to date in these species have detected no H5N1 HPAI viruses in breeding or juvenile birds. Experimental studies show that some mallard ducks continue to shed virus for up to 17 days, allowing the development of humoral immunity and subsequent selection of antigenic variants within the same bird. 23 If this occurs, it could be argued that a limited number of ducks would be sufficient to maintain the virus in nature. Continued surveillance is needed to determine whether H5N1 HPAI viruses are maintained in nature by a small number of naturally resistant ducks that are long-term virus shedders.