Gray's Grasshopper Warbler

Gray's grasshopper warbler, also known as Gray's warbler, is a small passerine of the family Locustellidae, formerly included in the 'Old World warbler' assemblage. The Sakhalin grasshopper warbler was formerly considered the same species. It breeds in southern Siberia and north-eastern Asia (Wikipedia, general species account). The source used here gives no further description.