Marsh warbler  (Acrocephalus palustris)

This rare summer migrant is olive green-brown in colour with a white throat, a pale stripe in front of the eye and pale legs (2). The song is extremely varied and musical; the marsh warbler is an excellent mimic and on average incorporates the songs of 31 European and 45 African species into its repertoire (2).

Patchily distributed in Europe, the breeding range of the marsh warbler extends mainly from eastern France to Russia, north to Sweden and south to Greece and Turkey. They migrate to south east Africa in winter (2). The marsh warbler was recognised as a British species in 1871, and is found mainly in south east England (2).

Breeding habitat in Britain tends to be damp areas (3), often near water (4) with scattered tree cover and plenty of herbaceous vegetation such as large clumps of nettles (4).

In common with all warblers, the marsh warbler is very active. It searches for its favourite invertebrate prey of small insects and spiders, which are picked from vegetation (2).

This species arrives back in its British breeding grounds between late May and mid-June, much later than its relative the reed warbler (2), and pairs soon set about building their cup-shaped nests. The nest is built around plant stems close to the ground. One brood of 4-5 pale blue eggs is laid in May to early July (3) and both parents share the 12 day incubation duties (2).

Following a decline in numbers between 1940 and 1960, the British marsh warbler population reached a stable level of around 100 pairs. This population then underwent a severe decline, until there was just one male recorded in the West Midlands in 1989 (5). A population became established in Kent during the 1970s, which had increased in numbers to 25 by 1993 (5). Habitat loss and climate change may have been factors in the decline during the 1950s and 1960s. Local habitat loss resulting from the activities of water authorities such as bank tidying is likely to have affected the species (3), as is disturbance by birdwatchers and egg collecting, both of which have been recorded. Climate change may continue to affect the population, as this species is at the northern extreme of its range in the UK (2).

Vulnerable sites have wardening schemes in place to prevent egg collecting and disturbance by birdwatchers, in addition most of the former breeding sites in Worcestershire are designated Sites of Special Scientific Interest (SSSIs). The Worcestershire Wildlife Trust has produced management guidelines for rivers, which will be promoted by the Environment Agency in areas where the marsh warbler occurs. The marsh warbler is a priority species under the UK Biodiversity Action Plan (UK BAP), which aims to encourage a long-term expansion of the population in the UK (5).

For more on British birds see the RSPB website:
http://www.rspb.org.uk

Information authenticated by the RSPB:
http://www.rspb.org.uk/