West Nile virus (WNV) is carried by birds, and in the wild, has only been known to be transmitted from bird to bird by mosquitoes. Birds in the "enzootic cycle" are relatively resistant to disease, and serve to maintain the virus in the avian population. Susceptible avian species and mammals may be infected when bitten by a "bridge vector" mosquito (one which will feed on birds and mammals), which essentially takes the virus out of the enzootic cycle. Birds in the family Corvidae-crows, blue jays, and ravens-have proven to be especially susceptible to West Nile virus (WNV) infection, and have been the focus of WNV surveillance efforts across the United States and Canada.

Research has shown that it is possible for bird to bird transmission to occur as a result of oral and cloacal secretions contaminating water and food. There may be a direct bird to bird transmission but the cause is unknown. Research has also shown that the virus may be passed to raptors by eating infected birds. However, the most likely route of transmission is still the mosquito.

Most infected corvids die within 3 weeks of infection. Clinical signs prior to death may include incoordinated flying or walking, weakness, lethargy, tremors, and abnormal head posture (shown in a great-horned owl on the right). Although crows and blue jays account for between 50 and 90 percent of reported avian cases, WNV infection has been identified in over 170 other avian species during the North American outbreak. The list includes over 130 native North American birds and nearly 40 exotic and zoo species. During the summer of 2002, significant numbers of raptor species (owls, hawks, eagles and vultures), especially in the midwestern United States, were found to be infected with the virus. Many mammalian wildlife species have also been shown to be positive for WNV. Domestic poultry (chickens, turkeys) and pet birds (budgerigars, cockatoos, cockatiels) seem fairly resistant to disease caused by WNV.