Marsh Warbler

The species is usually monogamous. Marsh warblers tend to choose new mates each year and do not necessarily return to breed in the same area as previous years. On their breeding grounds they are territorial, with territories often grouped into loose colonies. In Africa, they are essentially solitary, and may defend territories to some extent. The nest is a cup, made mostly from leaves and plant stems, and is usually in dense vegetation, at varying heights. Three to six eggs are laid. Both sexes bring food to the nestlings. In continental Europe at least, the species has a short breeding season, of 52–55 days. In some areas, such as Bulgaria, marsh warblers suffer significant levels of parasitism by common cuckoos.

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. Singing male are occasionally heard in Ireland, most recently in 2017. 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.

Globally, the population is believed to be increasing, and the IUCN categorises the species as of least concern. It is estimated to have a total population of 10 to 27 million individuals. The reasons for the population decline in Britain are not completely understood, and there appears to be much suitable habitat. The Biodiversity Action Plan for the species further comments that it is not clear what can be done to conserve the species apart from protecting habitat at known breeding sites and protecting birds from egg collectors and from disturbance.