Direct transmission of avian influenza viruses to humans has been confifi rmed and ever since it has been the main topic of influenza virus research. The opposite form of virus transmission is still unclear. In our study we used duck flocks as sentinels for surveillance of wild birds for influenza viruses and the possibility of bird infections with human influenza A viruses. Tested sera were collected from ducks in a free breeding system on a fish pond in the ornithological reserve Kopački rit (Croatia). Ducks were bred in an isolated unit for the first three weeks, and this was followed by four weeks breeding on the fifi sh pond without contact with humans. Sera were tested by haemagglutination inhibition (HI) test using human influenza viruses A/New Caledonia/20/99/ VR-116 (H1N1), A/Panama/2007/99 (RESVIR - 17) (H3N2), B/Hong Kong/330/01 and B/Sichuana/379/99 as antigens. To determine the time of infection, sera were collected twice during the period of isolation (day 0 and day 21) and at the end of breeding (day 49). Sera collected during the period of isolation were negative to influenza A and influenza B viruses. The high titer of influenza A specific antibodies in the serum samples at the end of breeding confirmed infection during exposure on the fish pond. As ducks on the fish pond had no contact with humans and were only in close contact with wild migratory birds these results confirmed that wild migratory birds were a source of infection and present a reservoir of influenza viruses. Very high seroprevalence in sentinel ducks with a high HI titers in some animals was determined at the end of breeding. The results of this study indicate that sentinel ducks were infected with influenza A virus strains closely related to the human strains used as an antigen. Also this study confirmed that sentinel ducks could be successfully used in influenza monitoring in wild birds.