Gray's Grasshopper Warbler

Gray's grasshopper warbler, Helopsaltes fasciolatus, is a small locustellid formerly included in the broad Old World warbler assemblage. Adults have an unstreaked olive-brown back, an even grey breast, buff underparts and dull orange undertail coverts without mottling. The song is a short, loud, musical phrase rather than the prolonged insect-like reel associated with European Locustella warblers. The Sakhalin grasshopper warbler was formerly treated as conspecific (Wikipedia, general species account).

The species breeds across southern Siberia, northeastern China and Korea and migrates to Southeast Asia for winter. It occupies lowland and coastal regions, placing nests in forest or dense thickets. Its plain upperparts and underparts help identification, but the distinctive song is especially useful where dense vegetation obscures the bird. Korean observations are most expected during the breeding or passage seasons; the opened account does not provide clutch, incubation, size or mass figures, so those statistics remain omitted (Wikipedia, general species account).