In order to investigate the prevalence and ecology of the Avian Influenza A virus (AIV) in western Palearctic wild birds, migrating ducks, mainly Mallards ( Anas platyrhynchos), have been caught in a funnel trap at the Ottenby Bird Observatory on the Swedish Island Öland. After banding and collection of biometrical data, every individual was sampled for AIV. Since the fall of 2002, more than 3200 samples from 1900 birds have been collected showing a prevalence of AIV of 12%. Several subtypes have been found, including the low pathogenic H5N2. Some ducks are re-caught both during fall and spring migration, and the recruiting and wintering areas of the birds are known enabling us to draw conclusions about the geographical and reservoir distribution of AIV.