The species was once widespread in East Asia, but large-scale exports and the destruction of its forest habitat have reduced populations in eastern Russia and China to below 1,000 pairs in each country; Japan, however, is thought to still hold some 5,000 pairs. The Asian populations are migratory, overwintering in lowland eastern China and southern Japan.(Cited reference :Wikipedia)
According to my observation, it is obvious that the number of migrating birds that come here from the north area is getting down every year.I don't know the reasons why, but one reason may be will climate-changing such as high temperature, rainfall here, especially in this year.