The sedge warbler (Acrocephalus schoenobaenus) is an Old World warbler in the genus Acrocephalus. It is a medium-sized warbler 11.5–13 cm (4.5–5.1 in) long and weighing around 12 g (0.42 oz) with a brown, streaked back and wings and a distinct pale supercilium. The plumage of the sexes is identical. Sedge warblers are migratory, crossing the Sahara to get from their European and Asian breeding grounds to spend winter in Africa. The sedge warbler is mostly insectivorous and has a 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. Male sedge warblers commence singing only a few hours after arriving on their breeding territory. The song is given from a bare perch such as a reed stem or bush, or from cover and during routine flights within their territory. Song-flights are also performed: while singing, the bird takes off, rises to a height of around 2–5 metres and then after a short circling flight, makes a slow, ‘parachuting’ descent, often with the wings held up in a ‘v’ shape. The song has the function of attracting a mate, rather than keeping other males away, and is stopped as soon as a mate is found. Contact calls are described as chirr or kerr; these calls are repeated quickly to form a rattling alarm call.
Sedge warblers feed in low, thick vegetation, especially reeds and rushes, but also in arable fields and around bushes.