The Marsh Warbler, Acrocephalus palustris, is an Old World warbler in the genus Acrocephalus. It breeds in temperate Europe and western Asia. It is migratory, wintering in south east Africa. It does not breed in the Iberian peninsula, and in England it is scarce and declining, with the former main centre of population in Worcestershire now extinct.
Today, in Kent, the former area of St Margaret's Bay that at its best peak held 17 pairs in 1993 is now deserted due to the activities of eggers and has been so since 2003, but birds persist on a private SSSI site in Sandwich Bay and possibly elsewhere in the Stour Catchment. In Essex, the bird is on the increase, breeding along the Thames corridor on brownfield, former industrial sites and also neighboring SSSIs and nature reserves; this nucleus of birds is found mostly in Eastern boroughs in the vicinity of Barking, Dagenham Dock, Rainham and the Ingrebourne Valley although exact sites are necessarily withheld. A handful of pairs breed in Norfolk and may still persist in Sussex, with further sporadic breeding in Yorkshire (2008), Tynside and SE Scotland.
This small passerine bird is a species found in fairly tall rank vegetation in marshes or by rivers. 3-6 eggs are laid in a nest in reeds or low vegetation. This species is usually monogamous (Leisler & Wink 2000). In breeding habitat in SE England today, it shows a strong preference for rank herbaceous vegetation in proximity to taller bushes and at two current breeding sites, favours rich wasteland with a profusion of rosebay willowherb, cow parsley and nettle.
This is a medium-sized warbler. The adult has a plain brown back and pale underparts. It can be confused with the Reed Warbler, but is greyer on the back, the forehead is less flattened and the bill is less strong and pointed. The sexes are identical, as with most warblers, but young birds are yellower below.
In the breeding season, the best identification feature is the song, which is high and fast, and consists almost entirely of mimicry of other birds, punctuated with typically acrocephaline sqeaks and whistles. Dozens of different European and African bird calls have been identified in the song of this warbler.
The reasons for the population decline of this species are not completely understood, but the fragmentation of habitat has been discussed in UK official documents of it Biodiversity Action Plan, a national program for protection of this declining species. Fragmentation is a generic term usually associated with land develoment patterns of an expanding human population. It is notable, however, that in continental Europe at least, the species in known to have a short breeding season (52-55 days: Leisler & Wink 2000) making it more vulnerable to climate change.