Wild birds of the ordersAvian influenza (AI) viruses have been reported from a wide diversity of free-living birds representing over 100 species in 12 taxonomic orders (11). All of the known hemagglutinin (H1 to H16) and neuraminidase (N1 to N9) subtypes of AI viruses have been isolated from wild birds (8, 9, 11), and currently, species in the orders Anseriformes (ducks, geese, and swans) and Charadriiformes (gulls, terns, and shorebirds) are believed to be the natural reservoirs. Surveillance for AI virus in wildbird populations is predominately dependent on diagnostic assays that identify the virus, including reverse transcriptase PCR and virus isolation. Virus isolation from oropharyngeal or cloacal swabs of embryonating chicken eggs currently represents the preferred method of AI virus diagnosis and surveillance in wild-bird populations. However, agent-specific identification assays, such as virus isolation and reverse transcriptase PCR, are expensive, labor-intensive, and dependent on the host actively excreting virus. Consequently, a limitation of the agent identification-based approach to AI virus surveillance in wild birds relates to the relatively short duration of viral shedding and the high degree of spatial and temporal variations in viral prevalence within different wild avian populations. These limitations and uncertainties often necessitate large sample sizes to identify positives and repeat sampling at different times and locations. Additionally, the variability creates difficulty when interpreting negative test results, i.e., in determining whether a negative result is indicative of inappropriate sampling (wrong location or time) or a species that is resistant to or rarely infected with AI virus.Serologic assays are commonly utilized for surveillance and diagnostics with domestic poultry to detect whether a population of birds has previously been exposed to an AI virus. Serologic tests utilized for AI virus antibody detection in domestic poultry include the agar gel immunodiffusion (AGID) test, the enzyme-linked immunosorbent assay (ELISA), the hemagglutination inhibition test, and the neuraminidase inhibition test (17). The AGID test and the ELISA detect antibodies against all type A influenza viruses and consequently are the preferred assays for use as a screening tool. The hemagglutination inhibition and the neuraminidase inhibition tests are hemagglutinin and neuraminidase specific, respectively, and typically are performed to identify antibodies to specific subtypes or to confirm AGID test-or ELISA-positive samples when information on the subtype is available.The AGID test is the most commonly utilized serologic assay for AI virus surveillance in domestic poultry and detects antibodies directed against the AI virus internal proteins nucleoprotein (NP) and matrix 1 (M1) protein (20). While the AGID test is inexpensive and simple to perform, the primary disadvantage is that it is only moderately sensitive for gallina-* Corresponding author. Mailing address: Southeastern Cooperative Wildlife D...