Highly pathogenic avian influenza virus (HPAIV) subtype H5N1 prevails worldwide and causes serious problems in poultry industry. The virus is also known as one of the most important zoonotic agents derived from avian species. Because many bird species other than poultry such as chicken and duck are susceptible for HPAIV infection, wild birds are thought to play an important role in distribution and transmission of the virus. However, the ecological role of wild birds as a reservoir of HPAIV in nature has not been completely understood. To define the ecological role of wild birds in distribution of HPAIV, extensive surveillance in wild birds including migratory and resident birds in Japan was conducted. Until now, 3 strains of H5N1 subtype have been isolated. One was isolated from mountain hawk-eagle (Spizaetus nipalensis) which was found sick at Sagara village, Kumamoto prefecture, Japan on January 2007 and ultimately died after a short while. The other two strains were isolated from whooper swans (Cygnus cygnus) which were found at Lake Towada in Aomori prefecture in April and May 2008, respectively. Because the wild birds migrate on a global scale, similar problems could be always happened in any other countries. Consequently, comprehensive surveillance in wild birds with international cooperation is required for efficient global control of HPAI.