抄録
The number of Pintails, Anas acuta, were counted when wintering at eight ponds around Tokyo, Japan. Two ponds were monitored weekly and six ponds were monitored monthly during the 2003-2004 wintering season (from November to February). Every pond was monitored monthly during the 2004-2005 wintering season. We found that: (1) the sex ratio of the wintering ducks varied between the ponds, (2) the sex ratios were generally biased and tended to be robust both within and between seasons, (3) the proportion of males was negatively correlated with the proportion of the ducks on land, and (4) females were observed more frequently on land than males.
