The Thick-billed Warbler, Acrocephalus aedon, is an Old World warbler in the genus Acrocephalus. It breeds in temperate east Asia. It is migratory, wintering in tropical south east Asia. It is a very rare vagrant to western Europe.
This is a large warbler, at 16-17.5 cm long nearly as big as Great Reed Warbler. The adult has an unstreaked brown back and buff underparts, with few obvious distinctive plumage features. The forehead is rounded, and the bill is short and pointed. The sexes are identical, as with most warblers, but young birds are richer buff below. Like most warblers, it is insectivorous, but will take other small prey items.
The song is fast and loud, and similar to Marsh Warbler, with much mimicry and typically acrocephaline whistles added.