UKBirds

Marsh Warbler

Acrocephalus palustris

The Marsh Warbler, Acrocephalus palustris, is an Old World warbler currently classified in the family Acrocephalidae. It breeds in temperate Europe and western Asia and winters mainly in south east Africa. It is notable for incorporating striking imitations of a wide variety of other birds into its song.

The Marsh Warbler breeds in a variety of mostly damp habitats, but in Africa winters mainly in dry, well-vegetated areas. It is common over much of its breeding range and expanding its distribution in some areas. However in Britain it is now virtually extinct as a breeding bird. This insectivorous warbler can be easily confused with several close relatives, but the imitative song of the male is highly distinctive.

This is a medium-sized warbler. It is very similar in appearance to several other acrocephaline warblers, such as the Reed Warbler which also occurs in wetlands and has a similar breeding range. The male's distinctive song is useful for identification, as no other member of the genus mimics other birds to any significant extent. The Marsh Warbler also tends to avoid the stands of pure reed which are the Reed Warblers' favoured habitat.

The species is monotypic, and there is no significant geographical variation. The sexes are alike in appearance. Hybridisation with both Reed Warbler and Blyth's Reed Warbler has been occasionally recorded.

The Marsh Warbler breeds in the middle latitudes of Europe and western Asia, from the English Channel to about 70 degrees east. It mainly occupies areas with a continental climate, but breeds, or has bred, in Britain and northern France as well. it is principally a bird of the lowlands, but occurs at altitudes of up to 3000m in Georgia. In recent decades it has expanded its range to the north, with increasing numbers of birds breeding in Scandinavia and north-west Russia.

In western Europe the Marsh Warbler breeds mainly in rank vegetation on damp or seasonally flooded soils, and is particularly attracted to tall herbaceous vegetation such as nettles, meadowsweet, willowherbs and to young osiers and other low woody plants. It may breed in urban brownfield sites with suitable vegetation, for instance in Berlin, and also occasionally in arable crops. In the eastern part of its range, it breeds on dry hillsides with shrubs and in open woodland, as well as the kind of damper habitats it frequents in the west.

The Marsh Warbler winters mainly in south-east Africa, from Cape Province north to Zambia and Malawi. It makes use of a range of well-vegetated habitats, from moist scrub to dense thickets and woodland edge, at altitudes of up to 2400m.

Marsh Warblers tend to migrate from Europe to Africa via the Middle East, with many crossing Arabia and arriving in Africa on Sudan's Red Sea coast. Adults usually leave their breeding grounds soon after their young are independent, with their offspring following about two weeks later. On the Red Sea coast most birds arrive from mid-August to mid-September, with numbers of adults peaking in August and of young birds in September. Birds tend to spend much of the autumn somewhere in north-east or east Africa, before continuing south to arrive on their wintering grounds in December or January.

In spring, Marsh Warblers leave their wintering grounds in March or April. They are thought to follow broadly similar routes to their autumn migration. Birds breeding in south-east Europe, for instance on the Black Sea coast, may arrive there by late April. In other parts of their range, the majority of birds do not arrive until mid-May. On the western and northern edge of their range, for instance in England, birds do not tend to arrive until the end of May or early June.

As a vagrant, the species has been recorded as far away as Iceland and Madeira.

In Britain the species was never widespread, and disappeared from many areas from the 1930s onwards. By the 1970s Marsh Warblers bred in significant numbers only in Worcestershire, where 40-70 pairs were recorded each year during that decade. This population was effectively extinct by the end of the 1990s. From the 1970s and 1980s onwards, a very small population slowly developed in south-east England. However this population is also now close to extinction.