Abstract
To examine the seasonal movement of Spot-billed Ducks Anas poecilorhyncha, fluctuation of the duck population was studied from October 2003 to April 2005 in Ono Reservoir, and from August 2004 to April 2005 in the nearby Katsura and Tsuru Rivers in Yamanashi Prefecture, central Honshu, Japan. The Spot-billed Duck population in the reservoir was highest during the breeding season from May to September, but the species was rarely observed during the wintering season from October to April. However, the remarkable decrease of the Spot-billed Duck population in winter was not found in the river. On the other hand, although Mallards A. platyrhynchos and other wintering ducks were observed in the reservoir, they were rarely observed in the river. Since interference by Mallards on Spot-billed Ducks has been reported, that could be one of the possible explanation of the observed phenomenon.
