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.
Taxonomy
The sedge warbler was formally described by the Swedish naturalist Carl Linnaeus in 1758 in the tenth edition of his Systema Naturae under the binomial name Motacilla schoenobaenus. The species is now placed in the genus Acrocephalus that was introduced in 1811 by Johann Andreas Naumann and his son Johann Friedrich Naumann. British ornithologists did not distinguish the species from the common reed warbler until the 18th century. The genus name Acrocephalus is from Ancient Greek , "highest", and , "head". It is possible that the Naumanns thought akros meant "sharp-pointed". The specific schoenobaenus is from Ancient Greek , "reed", and ', "to tread". It is a translation of the old Swedish name for the bird, Sävstigare. The species is considered to be monotypic: no subspecies are recognised. 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. 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. Global changes in population have not been measured, but the sedge warbler's status is designated 'of least concern' by BirdLife International.
thumb|right|Sedge warbler in its habitat: a reedbed. [[Uitkerke, Belgium.]] It breeds across Europe and western and central Asia and is migratory. After feeding up post-breeding, they migrate quickly across southern Europe and the Sahara from August to September. Studies in Nigeria and Uganda suggest that sedge warblers return to spend winter at the same sites, year after year.
All sedge warblers spend winter in sub-Saharan Africa, from Senegal in the west to Ethiopia in the east, and as far south as the eastern Cape Province of South Africa and northern Namibia. The routes taken on the southward migration, and their eventual wintering grounds, correspond to the birds' breeding grounds. Birds ringed in the United Kingdom and Netherlands are later found from south-west Iberia to Italy; birds from Sweden are recovered in central Europe and Italy; while Finnish birds are found in north-east Italy and Malta east to the Aegean region. Sedge warblers from the former Soviet Union take routes via the eastern Mediterranean Sea and Middle East. On the African wintering grounds, habitats such as reeds at wetlands, papyrus, grass, sedge and reedmace and tall elephant grass are used. It can be found at altitudes of above sea level in Ethiopia. Contact calls are described as chirr or kerr; and these calls are repeated quickly to form a rattling alarm call. A study in Nottinghamshire, England, showed that a third of males which returned bred within 50 m or less of their previous breeding site, and only 14% bred further than away. Males commonly practice promiscuity, and part of the males settle a new territory and resume territorial behavior whilst the female incubates the first clutch. When successfully pairing with another female the male will usually take part in rearing both broods.
thumb|Egg, Collection [[Museum Wiesbaden]]
If eggs or chicks are predated, the female will often leave the male, who then resumes singing and pairs with another female. 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.