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, there is a prominent whitish supercilium, the crown is streaked with black, and the bill is strong and pointed. The legs are greyish. This is a medium-sized warbler, 11.5-13 cm long and weighing around 12 g. 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.
Habitat and Distribution
Sedge warblers are migratory, crossing the Sahara to get from their European and Asian breeding grounds to spend winter in Africa.
Prey taken by sedge warblers includes mayflies, dragonflies and damselflies, grasshoppers, bugs, lacewings, moths, beetles and flies. Vegetable material includes elderberries and blackberries. On their wintering grounds food includes non-biting midges and flowers and berries from the toothbrush tree.
The nest, built by the female, is in vegetation on the ground or up to a height of 50 cm. The cup-shaped structure has an outer layer of grass, stems and leaves, plus spiders' webs, with a thick, finer layer inside including reed flowers, animal hair and plant down. It is woven around vertical plant stems. Between 3-5 greenish-yellow and brown-mottled eggs are laid, measuring 18 x 13 mm and weighing 1.6 g each. They are incubated by the female for 14 days; the chicks are altricial and naked. Both male and female care for the chicks, which fledge after 13?14 days. After leaving the nest, young sedge warblers continue begging for food from their parents for between 1?2 weeks after learning to fly.
Calls and Songs
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.