The sedge warbler (Acrocephalus schoenobaenus) is an Old World warbler in the genus Acrocephalus. It is a medium-sized warbler with a brown, streaked back and wings and a distinct pale supercilium. Sedge warblers are migratory, crossing the Sahara to get from their European and Asian breeding grounds to spend winter in Africa. The male's song is composed of random chattering phrases and can include mimicry of other species. The sedge warbler is mostly insectivorous.
It has a streaked brown back and wings, and pale underparts. The rump is warm brown and unstreaked, contrasting with the duller wings. The forehead is flattened, the crown is streaked with black, and the bill is strong and pointed. There is a prominent whitish supercilium. The legs are greyish.
The plumage of the sexes is identical, although they can be told apart when caught for ringing by the presence of a brood patch or cloacal protuberance. Juvenile birds have dark spots on the breast. They can be easier to confuse with aquatic warblers due to an apparent pale central crown stripe contrasting with the darker edges. Other similar species include moustached warblers and Pallas's grasshopper warblers. The oldest recorded sedge warbler was a bird ringed in Finland which reached the age of 10 years, 1 month. The typical lifespan is 2 years.
The song is varied, rushed and chattering, with sweeter phrases and some mimicry, typical of the Acrocephalus warblers. It is composed of phrases in random order, so that it is never the same. Male sedge warblers which have the widest repertoire mate with the largest number of females.