Long-term changes in the number of diving ducks wintering in Lakes Nakaumi and Shinji, Japan

Abstract

Many birds migrate during late autumn and stay until early spring at Lakes Nakaumi and Shinji in San-in district, western Honshu, Japan. The numbers of diving duck species Aythya ferina, Aythya fuligula, and Aythya marila, which constitute the majority of wintering bird populations, were determined monthly during the winter season from 1999 to 2015 in the main area of L. Nakaumi, Honjo Area, which is separated from L. Nakaumi by reclamation dikes, and L. Shinji. The mean value in December and January, obtained from changes in the monthly numbers of each diving duck in the three areas, was used as the wintering number. The long-term data showed a declining trend in the numbers of A. ferina and A. fuligula wintering in L. Nakaumi and Honjo Area. In L. Shinji, long-term data for A. fuligula also showed a decline trend, however, the data for A. marila indicated an upward trend. In L. Nakaumi and Honjo Area, the main diet for these diving duck species is the mussel Musclista senhousia, while in L. Shinji they feed on Corbicula japonica. The long-term changes in wintering bird numbers were investigated in relation to changes in food availability and habitat in the two lakes.