The bittern (Botaurus stellaris) is more likely to be heard than seen. The loud booming call, uttered by males during the breeding season, can be heard from up to two kilometres under suitable conditions. It is a secretive bird, its plumage subtly mottled in various shades of brown, which help it to blend with the reed stalks amongst which it lives. When startled it adopts a camouflage posture, bill pointing upwards and neck stretched vertically. In flight it resembles its close relative the heron but, in good visibility, is easily recognised by its colouration.
Bitterns feed on fish, especially eels, amphibians and invertebrates. They usually hunt along the reed margins in shallow water and on the edges of dykes.
Males are polygamous with each mating with up to five females. The nest is built in the previous year's standing reeds and consists of a platform some 30 centimetres across. The eggs are laid in late March and April, usually four or five in number, and incubation and care of the chicks is provided by the female bird. After hatching, the chicks spend about two weeks in the nest and then disperse amongst the reeds. They adopt the camouflage posture of the adult when threatened but little else is known about the chicks' feeding habits and behaviour. The birds usually leave their nesting grounds in winter but the UK population is often joined by over wintering birds from the continent. Due to its secretive nature, bittern numbers are usually monitored by counting the booming calls of males heard at the main sites.
Across most of their range, bitterns are a bird of shallow reed beds although in central Europe they breed in swamps dominated by reed mace or common bulrush. Bitterns need large, extensive areas of managed reed bed with shallow pools or dyke edges in which to hunt fish. Lack of suitable management allows scrub to develop and leads to reed beds drying out, one of the reasons that is thought to have led to the decline in bittern numbers in the UK.
The bittern is classified as Least Concern (LC) on the IUCN Red List (1). Included in the Birds of Conservation Concern Red List - (UK). Fully protected under Schedule I of the Wildlife and Countryside Act, 1981. Listed under Annex I of the Birds Directive, Appendix II of the Bern Convention and Appendix II of the Bonn Convention.
Much of the bittern's former habitat of large reed beds has deteriorated due to lack of management, loss to agriculture, pollution and in recent years, inundation of coastal sites caused by sea-level rise. By 1995 a survey of UK reed beds revealed only 45 sites with an area greater than 20 hectres. These remaining sites were also very localised and this fragmentation also reduces the bird's opportunities to breed in significant numbers. In East Anglia some of the bittern's most valuable breeding areas have begun to be seriously threatened by the sea. Two of the largest reed beds in Britain have suffered salt-water contamination, which kills off not only reed but also the food sources on which the birds depend.
Reed beds are one of Britain's most important habitats for birds, supporting four other rare breeding birds. These are the marsh harrier, Cetti's warbler, Savi's warbler and the bearded tit. They also provide valuable roosting sites for many thousands of migrating birds as well as being rich in plant and invertebrate life. As part of the UK Biodiversity Action Plan (UKBAP) English Nature has produced an action plan for reed bed birds in England as well as individual Species Action Plans (SAPs) for these five birds. The bittern is included in English Nature's Species Recovery Programme, which aims to restore existing reed beds back to favourable condition and to create new reed bed habitats both close to and away from traditional bittern breeding grounds. Where coastal reed beds have been identified as under threat from rising sea levels, new areas have been targeted for expansion and bunds built to impede inundation by seawater. In partnership with the RSPB, English Nature's Bittern Recovery Project has examined ways of turning agricultural land, also threatened by sea-level rise, into reed bed habitat through grants such as Countryside Stewardship.
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