1. Dept. of Natural History Sciences, Graduate School of Science, Hokkaido University, N10W8, Kita-ku, Sapporo 060-0810, JAPAN
2. Yamashina Institute for Ornithology, Konoyama 115, Abiko, Chiba 270-1145, JAPAN
3. Dept. of Natural History Sciences, Faculty of Science, Hokkaido University, N10W8, Kita-ku, Sapporo 060-0810, JAPAN