I guess a better example for the moment would be that birds do not migrate from NY to Montana, however,heath officials in Oregon are expecting the virus in their state any day now.
The process involves birds, mosquitos, and a virus. A single bird doesn't need to travel the distance. Several birds and mosquitos can. It's a small, small world.
Mosquitoes, largely bird-feeding species, are the principal vectors of West Nile virus. The virus has been isolated from 43 mosquito species, predominantly of the genus Culex (Table 1). In Africa and the Middle East, the main vector is Cx. univittatus (although Cx. poicilipes, Cx. neavei, Cx. decens, Aedes albocephalus, or Mimomyia spp. play an important role in certain areas). In Europe, the principal vectors are Cx. pipiens, Cx. modestus, and Coquillettidia richiardii, and in Asia, Cx. quinquefasciatus, Cx. tritaeniorhynchus, and Cx. vishnui predominate. Successful experimental transmission of the virus has been described in Culiseta longiareolata, Cx. bitaeniorhynchus, and Ae. albopictus (8,13). Transovarial transmission of the virus has been demonstrated in Cx. tritaeniorhynchus, Ae. aegypti, and Ae. albopictus, though at low rates.
Virus isolations have occasionally been reported from other hematophagous arthropods (e.g., bird-feeding argasid [soft] or amblyommine [hard] ticks) (Table 1), and experimental transmission has been observed in Ornithodoros savignyi, O. moubata, O.maritimus, O. erraticus, Rhipicephalus sanguineus, R. rossicus, Dermacentor reticulatus, and Haemaphysalis leachii (8,13).
Wild birds are the principal hosts of West Nile virus. The virus has been isolated from a number of wetland and terrestrial avian species in diverse areas (7-10,14-16). High, long-term viremia, sufficient to infect vector mosquitoes, has been observed in infected birds (7,17,18). The virus persists in the organs of inoculated ducks and pigeons for 20 to 100 days (18). Migratory birds are therefore instrumental in the introduction of the virus to temperate areas of Eurasia during spring migrations (12,14-16,19).
Rarely, West Nile virus has been isolated from mammals (Arvicanthis niloticus, Apodemus flavicollis, Clethrionomys glareolus, sentinel mice and hamsters, Lepus europaeus, Rousettus leschenaulti, camels, cattle, horses, dogs, Galago senegalensis, humans) in enzootic foci (8-10). Mammals are less important than birds in maintaining transmission cycles of the virus in ecosystems. Only horses and lemurs (20) have moderate viremia and seem to support West Nile virus circulation locally. Frogs (Rana ridibunda) also can harbor the virus, and their donor ability for Cx. pipiens has been confirmed (21).
Although Palearctic natural foci of West Nile virus infections are mainly situated in wetland ecosystems (river deltas or flood plains) and are characterized by the bird-mosquito cycle, argasid and amblyommine ticks may serve as substitute vectors and form a bird-tick cycle in certain dry and warm habitats lacking mosquitoes. Even a frog-mosquito cycle (21) may function under certain circumstances.
In Europe, West Nile virus circulation is confined to two basic types of cycles and ecosystems: rural (sylvatic) cycle (wild, usually wetland birds and ornithophilic mosquitoes) and urban cycle (synanthropic or domestic birds and mosquitoes feeding on both birds and humans, mainly Cx. pipiens/molestus). The principal cycle is rural, but the urban cycle predominated in Bucharest during the 1996-97 outbreak (2,3). Circulation of West Nile fever in Europe is similar to that of St. Louis encephalitis in North America, where the rural cycle of exoanthropic birds—Cx. tarsalis alternates with the urban cycle of synanthropic birds—Cx. pipiens/quinquefasciatus.