During the spring of 2004, approximately 1,950 blood specimens were collected from resident and Nearctic-Neotropical migratory birds on the Caribbean islands of Puerto Rico and Cuba prior to northerly spring migrations. Eleven birds and seven birds, collected in Puerto Rico and Cuba, respectively, showed evidence of antibody in a flavivirus enzyme-linked immunosorbent assay. Confirmatory plaque-reduction neutralization test results indicated neutralizing antibodies to West Nile virus in non-migratory resident birds from Puerto Rico and Cuba, which indicated local transmission.