The restoration potential of Aquatic warbler (Acrocephalus paludicola) habitats in the Lower Odra valley/Western Pomerania.

Summary

The Aquatic Warbler (Acrocephalus paludicola) is the only globally threatened passerine bird species in continental Europe. Around 1900, it was one of the most common birds in European fen mires. The world population collapsed in the course of the 20th century due to habitat loss caused by wetland drainage and agricultural intensification. Currently, the world population is approx. 12 500 singing males, of which the current core population (approx. 90%) breeds in Belarus, Eastern Poland and Northern Ukraine. The small and geographically isolated population in Pomerania (Northeast Germany/Northwest Poland; 80 singing males in recent years) represents the last remnants of a formerly large western Central European population and is threatened with extinction. Its conservation has high priority (reflected e.g. in the CMS Memorandum of Understanding of 2003).

The main objective of this study was to provide the scientific basis for the conservation and restoration of Aquatic Warbler habitats in Pomerania. In contrast to the well-studied core population, the poor knowledge of habitat requirements in Pomerania hampered conservation and restoration activities until recently. As it was assumed that the habitat deterioration in Pomerania is caused by vegetation succession, key vegetation and site condition parameters of habitat selection, currently best habitat conditions, and compatible land use techniques maintaining or creating such conditions were investigated.

Throughout the breeding seasons 2004-2006 all sites currently used by the bird in Pomerania as well as several recently abandoned and potential sites were studied. Data on vegetation structure (e.g. vegetation and litter height, vegetation density), soil and nutrient conditions, invertebrate composition and biomass, land use, and habitat heterogeneity were analysed with regard to the occurrence of singing males. In addition, female nesting site selection and foraging behaviour and the nestling diet (using a surrogate species) has been studied.

From the detailed analysis of all current breeding sites, it emerged that the less productive coastal breeding sites are characterised by dominant Phragmites australis stands with a well-developed herb layer (e.g. Rozwarowo Marshes, the stronghold of the species in Pomerania), and that the sites in lower Oder valley are more productive and dominated by Carex acuta, Phalaris arundinacea, or meadow grasses (e.g. Lower Oder Valley National Park, the last breeding site in Germany).

As in the core population sites, optimal conditions during late May/early June include a vegetation height of less than 70 cm, a cover of the lower herb layer of approx. 20% and of the upper herb layer of less than 60%, i.e. rather sparse vegetation. In contrast to the core population sites, where the water height is up to 20 cm and a thick litter layer is needed for building nests above it, in Pomerania the water height is only 0-1 cm and the thickness of the litter layer less than 10 cm. This is probably connected to the larger invertebrate biomass on moist sites with a small litter layer. Again in contrast to the core population sites, habitat heterogeneity is high and specific foraging habitats with a vegetation height gradient (edges between used and not used areas, ditches) and with moister conditions (ditches, depressions) are preferred by females provisioning their nestlings. The distance between foraging habitats with sufficient food supply and nesting sites cause (probably unfavourably) long foraging flights, but still allows successful breeding. A refined analysis of nesting site conditions and breeding success as well as of population development and exchange between breeding sites is recommended.

Trophic conditions in current and potential Aquatic Warbler habitats strongly influence the management needed to maintain or restore suitable conditions in Pomerania. Early summer land use is needed in the more productive sites to prevent habitat deterioration by succession to higher and denser vegetation. As this also poses a serious threat to broods, it is recommended to create a mosaic of early and late sed patches by alternating land use. Such a mosaic also offers edges as preferred foraging habitats. In the less productive sites, winter mowing can maintain suitable habitat conditions. Small stripes should be left uncut to increase prey availability and to provide nest-building material. Further eutrophication needs to be prevented, and habitat restoration is probably best supported by summer mowing. Such land use on mires and re-wetted peatlands belongs to a variety of wet, environmentally harmless forms of peatland agriculture and forestry (so-called paludiculture) that offer climate, biodiversity, and socio-economic benefits.

Study area

All current and several recently abandoned breeding sites of the Aquatic Warbler on the German and Polish side of the lower Odra Valley.

Status

Finished PhD study

Literature

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