Only certain vertebrates (called ‘amplifying hosts’) can retransmit West Nile virus (WNV) infection to arthropod vectors. Most competent amplifying hosts are birds. Competence is determined by the magnitude and duration of viremia. Virus concentrations in serum above 100,000 pfu per mL are required for infection of Culex pipiens and Culex quinquefasciatus . Avian species within the orders Passeriformes, Charadriiformes, Falconiformes and Strigiformes are usually competent. Environmental factors determine which competent species are important amplifiers within a transmission focus. Serologic data may be combined with bird relative abundance and competence to identify important amplifying hosts. Such data implicated the house sparrow (Passer domesticus) in New York and several passerine species including house sparrow, northern cardinal (Cardinalis cardinalis ), blue jay (Cyanocitta cristata ) and northern mockingbird (Mimus polyglottus) in Slidell, Louisiana. Birds are also important as sources of WNV infection for other birds in the absence of vectors, and as dispersal vehicles during migration.