The range of coots in Japan has increased in the past 50 years. In the 2000s, coots overwinter in almost the whole area of Japan and breed in every region of Japan except for Shikoku.

The number of wintering coots at Lake Biwa in Kinki has markedly increased in recent decades, especially from the end of the 1990s. Similar trends have been reported in several locations, such as Lake Kasumigaura in Kanto, Yamaguchi Pref. in Chugoku and also in South Korea; however, wintering coots at some sites in China and Hong Kong were reported to have decreased in the 2000s.

It is reported that coots with an attached wing-tag at Lake Kitaura, part of Lake Kasumigaura, in winter were recovered in Tohoku. The Bird Ringing Centre of Russia has records of two coots that were ringed at Lake Khanka in far-east Russia and were recovered outside Russia; in Vietnam and Kyushu in Japan. These records suggest that wintering coots in western Japan come from far-east Russia, and coots in Kanto comes from northern Japan. It is thought that the increase of coots in western Japan and Korea corresponds with the changes in China, but we have no information on why wintering coots in areas from Kinki to Kanto have so markedly increased. Breeding numbers in far-east Russia and north-eastern China might have increased. Further study is needed on coots migration and environmental changes in their habitats.