The breeding range is restricted to the western Palearctic between 47° and 59°N. Breeding occurs in Germany, Poland, Hungary, Lithuania, Latvia, Belarus, Ukraine and Russia. Possible breeding in Romania and Bulgaria has not been confirmed for the last 40 years. Population figures are given in Table 1.

 Table 1. Range states.

The breeding distribution is fragmented because of habitat constraints. The species became extinct in western Europe during the 20th century and has declined dramatically in central Europe. It formerly bred in France, Belgium, Netherlands, former West Germany, former Czechoslovakia, former Yugoslavia, Austria and Italy (Cramp 1992).

Recent studies on genetics and on stable isotopes in Aquatic Warbler feathers show that the German/north-west Polish population is genetically separate from all other studied populations (Gießing 2002), and that it has most probably a different, very restricted and more northerly wintering area than the other central and east European populations (Pain et al. 2004 and unpublished).  This sub-population is sharply declining, and is thought to be the last remnant of the formerly huge north German population. The west Siberian population is geographically completely separate, is most likely genetically separate too, and is probably headed for extinction.  In respect of these two sub-populations therefore it is likely that there will be a partial extinction of genetic variability within the species.

Aquatic Warblers have been recorded on migration in 13 European countries (Tab. 2). Birds from Poland, eastern Germany, and probably the whole Belarusian and Ukrainian Polessye migrate on a westerly heading along the Baltic coast in Latvia, Lithuania, Poland and eastern Germany, then along the North Sea coast of western Germany, Netherlands, Belgium and sometimes England, thereafter heading south along the French and Iberian Atlantic coast (Schulze-Hagen 1993, Aquatic Warbler Conservation Team 1999).

Table 2.  Main passage countries of the Aquatic Warbler in Europe. 

 Scattered records are known from the Mediterranean (as prey in nests of Eleonora’s Falcon), from Bulgaria and North Turkey (Kirwan 1992) so that it seems possible, that there is another, much less frequented flyway along the Black and Mediterranean Seas. 

Global distribution of the Aquatic Warbler ( Flade, 2008)

Aquatic Warbler: average global population 1996-2007 (singing males)

Map of European breeding sites of the Aquatic Warbler (AWTC, 2005)

The winter quarters lie in West Africa south of the Sahara. The only known bigger wintering site is situated in the Senegal delta (Djoudj National Park and grass marshes near Tiguet) and was discovered by an AWCT expedition in January 2007. There might be more wintering sites in wetlands and floodplains of Mauritania and Mali (several winter records), but further intensive search for suitable wintering sites in S-Mauritania and Senegal by the AWCT in 2008 remained unsuccessful (Flade et al. in prep.).

 Table 3. Number of records of Aquatic Warblers in wintering sites before 2007 (Schäffer et al. 2006). 

 The density of wintering AW in the grass marshes of the Senegal delta was estimated at 0.5-1 (-1.5) birds per hectare (but in a small area AW was much more abundant). The total area of suitable AW habitats (within and outside the Djoudj National Park) is estimated at 13,000 hectares, out of which 10,000 hectares might be suitable (water-logged) in late January/early February. Thus, the population estimate is not less than 5,000-10,000 birds. Since the global population actually is estimated at roughly 25,000 birds, the Senegal delta probably holds at least 20 %, eventually up to more than 60 % of the global population.

Once widespread and numerous in fen mires and wet meadows throughout Europe, the Aquatic Warbler has disappeared from most of its former range. Nowadays, its world population of only 10-14,000 vocalizing males is confined to fewer than 40 sites in only 8 countries, with 4 sites supporting over 80% of the world population (Tables 4 and 5).

Table 4. Breeding population size and trend by country (figures indicate the maximum number of singing males)

Table 5. Migrate or non breeding population size by country (figures indicate the maximum number of singing males)

Aquatic Warbler: development of the global population 1996-2007

Aquatic Warbler: development of the Pomeranian population

1996-2007

Aquatic Warbler: development of the Hungarian population

1996-2007

The Aquatic Warbler is a specialist of large open sedge and Cladium fen mires, which has suffered a very severe decline in western and central Europe due to habitat loss. These losses were caused mainly by drainage measures in fen mires and floodplains in order to enable or to intensify agricultural use, and for peat extraction. Also changes in the hydrological regime of the landscape (like channelizing and deepening of river beds, pumping stations etc.) had a severe impact. Other habitat changes like agricultural abandonment and uncontrolled burning became important only after fundamental changes of the general hydrological regime of suitable wetland habitats, but play a major role today. In the wintering sites, habitat losses through creation of fresh water reservoirs and increase of hydro-agriculture (rice and sugar cane crops) are of severe importance.

Change in hydrological regime in key sites

Some Aquatic Warbler breeding sites for example in Belarus and Ukraine suffer under unfavorable man-made changes in the hydrological regime.  This can lead for example to (1) lack of water, leading to reduced breeding success and population decline, (2) summer flooding with destruction of nests and (3) vegetation succession and loss of Aquatic Warbler habitat. In Ukraine, recently the deepening of parts of the upper Pripyat river channel resulted in a lower water table in the adjacent floodplain sedge mires and abandonment of AW breeding sites. In Senegal, the major wintering site of the global AW population is completely dependent on artificial flooding through the local water management facilities.

Importance:   critical

Breeding habitat changes due to abandonment

This is an important factor in Poland (Biebrza and Lublin marshes, NW-Poland), Lithuania, Russia (Kaliningrad region), Belarus (Zvanets, Sporova), Ukraine (huge areas in the upper Pripyat region) and Germany (lower Oder valley) where, if cutting of vegetation and/or burning (or in some places grazing) ceases, succession takes place and the habitat becomes unsuitable due to overgrowing by dominant sedge/grasses, high reeds, willow bushes or succession forests of birch or alder. In the past, reeds used to be occasionally harvested with scythes in the Biebrza and Zvanets mires, and along Yaselda, Stochid and upper Pripyat rivers for use on floors and for thatching, etc., together with the more important harvesting of sedges as poor quality hay and the active elimination of bushes. These traditions have now stopped at many places.

Importance:   critical

Overgrowth of  Sporava  mire 

Photo:  Alexander Kozuln

Habitat changes and habitat loss in wintering areas

Large formerly (most likely) suitable habitat areas in N-Senegal and S-Mauritania have been recently lost through transformation in fresh water reservoirs (e.g. Diama reservoir at the lower Senegal river, Keur Macène in Mauritania) or in irrigated hydro-agricultural crops (rice, sugar cane; e.g. S of Richard-Toll in Senegal).

Drought and habitat alteration in the winter quarters could be additional bottlenecks for the Aquatic Warbler. Main threats which have been identified are: drying up due to periods of drought; overgrazing of grasslands by cattle; succession of grass associations into scrub; increasing desertification as well as salinisation of irrigated soils. 

Importance:   high, potentially critical 

Loss of breeding habitat including drainage and peat extraction

This is usually related to drainage for agriculture or peat extraction/excavation, damming of floodplains (Pripyat, Yaselda) and also unfavourable water management (e.g. water extraction or drainage of adjacent areas) and canalisation of rivers. Currently there are problems at several sites in Poland, Belarus and Ukraine with drainage amelioration and peat extraction affecting adjacent sedge fens (upper Pripyat, Zvanets, Dikoe, Sporova) and also direct destruction still of fen mires in Ukraine (Volyn and Rivne regions).

Importance:   medium, but locally critical 

Breeding habitat changes due to uncontrolled burning

Burning is often used as a management tool in pastoral agriculture. Uncontrolled fires, especially in spring and summer and if the mire is very dry, cause severe habitat destruction by burning out of the upper peat layer. In Biebrza there was in 1994 a 3,000-ha fire which caused a great deal of soil mineralisation, but uncontrolled burning is more often a direct threat, especially (to birds and nests) during the breeding season. Big spring and summer fires happened also in the Zvanets and Yaselda mires in Belarus. In Hungary, burned areas of suitable habitat were reoccupied by Aquatic Warblers only 5-6 years after fire. - But note: controlled burning in winter or early spring can be an appropriate management technique for maintaining the habitat quality. 

Importance:   high

Result of uncontrolled burning

Photo:  Mikhail Maksimenkov

Eutrophication by waste water

Eutrophication of floodplain fen mires from city waste water and fish breeding ponds, by inundation with polluted river water, leading to changes in vegetation structure and species composition and speeding up the rate of vegetation succession, was observed in the Yaselda floodplain downstream of Berioza (Belarus). 

Importance:   medium 

Eutrophication resulting from mire drainage

Mineralisation of mires due to lowered water levels leads to minerals being washed downstream to flooded Aquatic Warbler areas, thus speeding up the rate of vegetation succession. This could be an important factor (Yaselda incl. Sporova mires, Belarus; several areas in the upper Pripyat region, Ukraine). 

Importance:   medium 

Road building

Habitat loss and alterations in hydroregime, due to building of roads on dams crossing fen mires and floodplains has occurred at some sites (e.g. Dikoe mire in Belarus) or is still planned (lower Oder in Germany). 

Importance:   overall low, but locally high

Unsuitable management by cutting or grazing

Some sites in Hungary, NW-Poland, NE-Germany (Swina delta and lower Oder valley) and Lithuania (Nemunas/Neman delta) suffer from too frequent/too intensive cutting or grazing, or from unfavourable agricultural management measures during the breeding season (too low intensity of cutting or grazing is included in the earlier section on ‘abandonment’). 

Importance:   low, but locally high 

Extremely early mowing in Nemunas Delta  

Photo:  Zydrunas Preiksa

Disturbance caused by man

In Biebrza birdwatchers tend to leave the paths and trample around the nesting habitat. This is also a potential problem in smaller places such as Chelm marshes. There have also been some attempts at egg-collecting but these have generally been prevented. 

Importance:   low and localised 

Habitat change and loss at migration sites

The extent of this problem and its impact on the population are unknown but could be affecting the population. 

Importance:   unknown