Land management for bitterns
The bittern is a shy species, favouring wetlands dominated by common reed (Phragmites australis).
About the bittern
Bitterns feed on fish and amphibians, which they get from within the reedbed or within the reeds’ edge.
They rarely venture out into the open. The bittern is one of our rarest breeding birds, currently occupying only about 30 sites in England, with overwintering birds at 50 or more sites.
The UK breeding population fell by more than 70 per cent between the 1950s and 1990s, largely due to loss of reedbed habitat and lack of management, as well as the drying out of existing reedbeds.
Considerable effort by the RSPB and Natural England, in partnership with others, has begun to reverse the changes. Numbers have increased from a low point of 11 booming males in 1997 to 76 in 2008.
What do bitterns need?
Throughout the summer, bitterns need:
- Large, undisturbed wet reedbeds, with good fish populations.
- Extensive undisturbed wet reedbeds (at least 0.2 sq km) or wetland complexes to provide cover from February to August.
- Good fish populations (preferably of rudd, eels and sticklebacks) for feeding.
Bitterns nest from April to June, but males may establish their territories as early as February. The males’ territories and females’ nests are found in reedbeds or tall fen vegetation, usually with standing water in and around it.
Adults feed within the reedbed and chicks are fed predominantly on any fish that venture into this area of wetland vegetation. In winter, wet reedbed or tall vegetation, with good fish populations, is required.
Any size of wet reedbed or tall vegetation, with good fish populations for feeding, offers bitterns cover through the winter. Many bitterns come to the UK from colder northern European countries from October to March.
Our own breeding birds and the additional continental visitors may occur at a greater range of smaller wetland sites with reedbeds or reed fringes in the winter months. These sites need good fish populations and some tall vegetation with standing water in which the birds can feed. Flowing water is advantageous in maintaining open conditions in severe weather.
How can I encourage bitterns?
On existing reedbeds
- The average territory size of British male bitterns is 0.2 sq km. If your site is part of a wetland complex, smaller areas of reed may be used. To encourage male bitterns to set up breeding territory, it is best to have 20–30cm of water in the reedbed by February.
- Female bitterns nest in freshwater reedbeds that have standing water of 20–30cm, without too many fluctuations in the depth. Many sites may have a natural draw-down of water from June to August. Females may still nest successfully in such sites if alternative suitable feeding areas within a kilometre remain wet.
- The reedbed should have at least 30 per cent open water with plenty of reed edge (250–450m of edge to each 0.01 sq km). It is important that this reed edge is such that water can flow freely into the reedbed (ie not a steep, dry lip as can occur where dredgings are heaped beside dykes). This allows fish to penetrate the area, offering food for bitterns.
- Check what’s there: drier reedbeds, perhaps with much scrub, are not ideal for bitterns, but may be home to rare plants, invertebrates, amphibians or birds, such as marsh harriers and bearded tits. The value of all sites should be checked before changing the way they are managed.
- Raise water levels: to make a dry reedbed wetter, it may be possible to raise water levels. However, doing only this may not be enough. Often there is a need, if judged acceptable, to clear invasive scrub as it may not be killed by raised water levels. The retention of some scrub will encourage the presence of invertebrates and other species.
- Lower reedbed: more extreme solutions involve lowering the reedbed by scraping off the top layer (at least 30cm). Sufficient reed rhizomes (root systems) should be left in the soil to allow the reed to recolonise quickly but, if necessary, these can be replaced. If there is insufficient edge or open water on-site, pools and channels should be created as part of this process.
- Management: once established, management of the reedbed will help reduce drying. This may entail cutting not more than 30 per cent of the reed cover each winter and removing the waste material. Ditches and channels with reed growing into and through them make good feeding places for bitterns. These should only be cleared out when fish can no longer use them (perhaps after 15 years or so).
- Do not leave any dredgings alongside the ditch because, as mentioned above, these will gradually form a lip along the edge and stop water and fish from getting into the reedbed. Try to place the dredgings in discontinuous sections, at least 20m in from the water’s edge, or remove them completely if possible.
- Encourage fish: encourage elvers (young eels) into your site by providing them with access. This could be a trickle of water out of the site on to an old carpet or some other kind of rough substrate they can climb up. Elvers usually run in spring and early summer. Encourage a healthy rudd population by promoting a good growth of submerged aquatic plants in the water column.
On water bodies with reed fringes
- Attract overwintering bitterns: if your site is under 0.2 sq km or the total amount of wet vegetation adds up to less than this, the work you do may still provide important habitat for overwintering bitterns. Existing bodies of water may already have a good fish population and bitterns can be attracted by the provision of good reed fringes – even small ones. The addition of even a small area of reedbed may attract overwintering bitterns.
- Pulling back the water edge, or filling in some deeper areas – depending on the size of the site – could provide shallow edges where reedbeds can be established. Newly planted reeds or growing rhizomes may need to be fenced off to protect them from grazing geese, coots or livestock.
- Reduce tree cover: many established bodies of water have edges that are overshadowed by trees and shrubs. You can enhance reed and other aquatic vegetation by cutting back, coppicing or pollarding adjacent trees.
Creating new reedbeds
- Aim for 30 per cent open water within the reedbed, including at least one large, deep (at least 1.5m deep) body of water to support a sustainable fish population. It is desirable to have a throughflow of water.
- The water body could be designed in a ‘hand’ shape, with wide ditches emanating from the main pool to provide plenty of wet reed edges for fish and bitterns to use.
- Gradually sloping substrates at the edges will help create a variety of water depths. An irregular design is encouraged – for example, an uneven scalloped edge to ditches is beneficial to fish and thus more likely to create feeding ‘hotspots’ for bitterns. The wet reed margin should be as wide as possible – 20–30m is ideal.