State

Once widespread and numerous in fen mires and wet meadows throughout Europe, the Aquatic Warbler has disappeared from most of its former range. In the last century, 95% of all aquatic warblers have gone extinct. The global population continues to shrink. There are only 4 countries left in the world where these rare birds regularly breed and nest – Belarus, Ukraine, Poland (containing 97% of the global population) and Lithuania. Several years ago, the warblers were able to nest in Germany, Latvia, and Hungary, Russia. Possible breeding in Romania and Bulgaria has not been confirmed for the last 40 years.

According the latest estimates, 2024, the global population is around only 8,000 singing males*.

County	Share of global population
Belarus	35%
Poland	34%
Ukraine	28%
Lithuania	3%

 

*The rough estimate by dr. Martin Flade, Global status of the Aquatic Warbler: Breeding grounds and spring stopover sites, presentation on LIFE4AquaticWarbler kick off meetin in Hungary, 2024,

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). Two small geographically isolated and genetically separate subpopulations of the Aquatic Warbler exist in Germany/northwest Poland and West Siberia (Russia). These populations are threatened with extinction. The breeding range is restricted to the western Palearctic between 47° and 59°N. The breeding distribution is fragmented because of habitat constraints.

The Aquatic Warbler (Acrocephalus paludicola) is Europe’s rarest songbird, the rarest and the only globally threatened passerine bird found in mainland Europe.

The aquatic warbler population is at least 20 times smaller than the population of African elephants.

Threats

• In the XX century 95 % of aquatic warbler population went extinct mainly due to habitat loss (drainage, abandonment, intensive use of fen mires and alluvial grasslands). Even though previous conservation efforts in various countries created greater quality in some important Aquatic warbler habitats, the loss of habitats remains one of the main threats for the bird. Habitats disappear due to intensive farming, change of land use, drainage of wetlands as well as abandonment of the land.

• In some areas where habitat conditions are favorable for the bird, populations are too small and in natural conditions are not capable of recovery.

• Another huge problem is habitat fragmentation. Those territories that are favorable for the species are too far from each other and small populations are isolated. This situation creates vulnerability and a huge threat for extinction.

• Water level is a crucial factor for the life of Aquatic warbler. It influences both home and food of the bird. Water level changes vegetation structure and can influence the quantity of invertebrates.

• In the key habitats for aquatic warblers, farmers play a mayor role in maintaining the habitats. Special agro-environmental schemes provide compensation for farmers that feel losses due to late mowing of those fields. Nevertheless, biomass utilization remains the biggest problem.

Population distribution

Global breeding area covers 375 km2

Species Conservation​

• The species is classified as vulnerable at global level and is listed as “Vulnerable” in the IUCN Red List of Threatened Species.
• At the European level it is classified as Endangered.
• Included as a priority species into Annex I of the EU Wild Birds Directive.
• Included in Appendix II of the Bern Convention and in Appendix I of the Bonn Convention.

• 22 countries signed the Aquatic Warbler Memorandum of Understanding, MOU, under the Bonn Convention.

State of the Population in Different countries

Belarus

A significant part of the global population of aquatic warblers breeds in Belarus.

Before the massive drainage of mires in 1960-s, the aquatic warbler was nearly continuously distributed in the central part of its breeding range, in the Paliessian lowland. There were c. 99 big fen mires (each of 1 000 ha and more) covering a total area of more than 900 000 ha. The mires were either adjacent to each other, or were located in a distance of 20-40 km. For that period the number of Aquatic Warblers is estimated at about 450 000 singing males.

At present there are only 3 key aquatic warbler breeding sites (Zvaniec, Sporava and Dzikoje) left in Paliessie region, holding c. 2 900 – 5 400 singing males (years 2010-2013). The rest of the national population breeds at 5 permanent sites with low numbers (Dzivin, Prostyr, Ščara, Servač and Svislač) resulting in 100 – 200 males and at 7 suboptimal sites holding 50-80 males in total. So, altogether 15 breeding sites were hosting 3 100-5 600 singing males in the years 2010-2013. Most of Belarusian breeding sites located at considerable distance (50-260 km) from each other. Such remoteness lowers the chance of population exchange between sites.

​Due to habitat degradation aquatic warbler numbers in Belarus are rapidly declining. At most of the sites, including the key sites Zvaniec and Sporava, the population decline is caused by fragmentation and an overgrowth of open fens with reeds and bushes. Some other sites (Dzikoje, Servač, Aĺmany and Žadzien) are on the way from fens to bogs. In 2017 the population of the country was estimated at 2787-3199 singing males. Later researches show the decease of the population.

Ukraine

A significant part of the global population (about 33 %) of aquatic warblers breed in Ukraine. According to the data of 2021, there were 2776-3205 singing males in the country. From 2010 the population is decreasing.

In 2016 a research was conducted to estimate the population of aquatic warblers in Ukraine. The research was done in Desna-Dnipro and Prypiat, which are two main brooding areas of this rare species. The research in Desna-Dnipro sites shows that the estimates of Aquatic Warbler population decreased, due to the fact that the environmental conditions of the habitat this year are worse compared to the conditions observed previously. The number of singing male aquatic warblers there currently range between 153-220.

In the Prypiat brooding area the population of aquatic warblers is also decreasing due to deterioration of key habitats caused by missing spring floods and habitat burnouts. This results in a range of 1857-2095 male aquatic warblers.

Reduced populations of aquatic warblers in both brooding sites resulted in a decreasing trend of total population. Both breeding sites combined comprise the total Aquatic Warbler population of Ukraine which is estimated to range between 2010-2315.

 

Bellow you can find dynamics of Aquatic Warbler populations in two main habitat groups.

Poland

The Polish breeding population represents almost 25% of the global population. According to the data of 2019, the population size is 4 500 singing males.

Except for the relatively stable large population in the Biebrza valley in north-east Poland little was known about population trends of peripheral populations in western, central and south-eastern regions of the country. A study in 2016 revealed that all small populations of Aquatic Warblers showed a significant decline in total number of individuals and declining number of populations over time. However, population trends were distinctly different in the different regions, with stable dynamics in south-east, moderate decline in north-east and sharp decline in the central and western regions. During the study period (1969–2013) 19 out of 38 populations became extinct (11 populations in the western region, two in central region, four in north-east region and none in the south-east region). Five of these populations were later recolonised thus suggesting a pattern of metapopulation dynamics. To mitigate the negative trends and increased risk of local and regional extinction in the western and central parts of Poland effort should be put to increasing dispersal among populations by increasing the number of stepping stone patches between the viable large eastern populations and the smaller central and western ones.

Lithuania

Lithuania is at the norther border of the Aquatic warbler territory. The population here is relatively small. In recent years an increase in the population was recorded after a decade of constant decrease. Scientists say that population is stabile now. Nevertheless, the size of population still fluctuates. Scientists link this to huge isolation of populations, lack of good.quality habitats and unstable conditions in breeding sites. Currently, the population in the country is stable and is around 200 singing males. In 2024, 240 males have been counted. More detailed information in PDF.

Germany

Even though a few aquatic warblers were noticed in Germany (on the Polish border a few years ago), and the country still has good quality habitats in Pomerania region, no aquatic warbler was found in 2017. In 2024 aquatic warblers has officially disappeared from Germany.

Hungary

Aquatic warblers have not been spotted in Hungary from 2011, when a mysterious extinction happened. Scientists still can not explain this sudden extinction. Rare birds were discovered in Hungary in 1971 in the only site in Hortobágy National Park.

Latvia

In Latvia last breeding season of Aquatic warbler was recorded in 2000-2002. Earlier Latvia was northern border of Aquatic warbler’s breeding area, but because of habitat loss and increasing fragmentation of habitats it was observed that Aquatic warbler do not come to this area for nesting purposes.

Russia

Last breeding season of Aquatic warbler in western Siberia was recorded in 2000. During intense research of Siberian mires in 1999-2001 three very small populations, consisting of 2-3, 1 and 8 singing males, were observed. Now it is thought that these populations might be extinct.

 

Breeding	Migration	Wintering
Belarus Poland Ukraine Lithuania Russia?	The Netherlands United Kingdom Belgium France Spain Portugal Morocco	Senegal Mauritania Mali

Biogeographic populations

Considering the geographical isolation of subpopulations and the results of the DNA studies of Gießing (2002), the following biogeographic populations was separated:

1)      the central European core population, including Belarus, E-Poland, Ukraine and Lithuania (c. 12,000 males previously);

2)      the isolated Hungarian population (60-700 males previously, now extincted);

3)      the Pomeranian population, including the NW-Polish and the German population (c. 80 males, now 0 males); this subpopulation is genetically distinct and obviously isolated since decades, and shows first signs of inbreeding depression (increased occurrence of homozygosity);

4)      the W-Siberian population, which is isolated from the core population by 4,000 km distance and is thought to become extinct (population estimate from the year 2001: 50-500 males, now – now information).

Genetic studies

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.

 

Migration

The path of migration of the aquatic warbler is not fully understood. 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). It is known that a large proportion of the population flies to West Africa for the winter, meaning they fly 6000 km two times a year. Migration begins in July-August. In 2020 the results of research have revealed that Lithuanian and northern Belorusian Aquatic Warblers are wintering farther than expected – they rest in Senegal, but then fly deeper into the Africa and settles in Mali. According current data, Mali seems to be the most important wintering country.

Aquatic Warbler fly partly over hostile areas, where the opportunity for resting or feeding is small (in the Sahara, for instance). If important stopover sites disappear, the aquatic warbler will face the immediate problem of survival. Imagine if the gas stations were removed along an entire highway. What would happen?

On migration the Aquatic Warbler has been recorded in 13 European countries, mainly in the west and southwest of the continent. The species winters in West Africa south of the Sahara. The regular wintering sites known so far are situated in the Senegal delta in and around the Djoudj National Park (Senegal), the Inner Niger Delta – a large complex of lakes and islands in the arid Sahel region, central Mali – and it’s surroundings. Mali seems to be the most important wintering country. unfortunatelly, the political situation there is unstable, making serious environmental work difficult.

Habitat requirements

The Aquatic warbler is a habitat specialist. During the breeding season it occurs mainly in sedge fen mires and similarly structured marshy habitats with a preferred water depth of 1–10 cm. In primeval landscapes it depends probably on mesotrophic or slightly eutrophic fen mires which stay open because of their surface oscillating with the river water table.

1.      Rich floodplain marshes in river valleys, comprising open sedge marshes with medium and large tuft-forming and scattered Carex (e.g. Biebrza and lower Oder river marshes in Poland, upper Ukrainian Pripyat), partly with taller Molinia caerulea or scattered, low stems of Phragmites australis, and often also scattered bushes, which all serve as singing posts for the males; this type of habitat depends more or less on human management (cutting or burning).

2.      Mesotrophic or poor eutrophic open sedge fen mires, the ground covered by green mosses; the grassy vegetation is dominated by low or medium, partly tuft-forming sedges (mainly Carex elata, C. diandra, C. rostrata, C. omskiana, C. juncella, C. appropinquata, C. lasiocarpa) and cotton grasses (Eriophorum angustifolium, E. gracilis), shallow water or wet pillows of mosses (Dikoe and Yaselda, Zvanets and upper Pripyat marshes, Uday, Supoy, Biebrza, Žuvintas); avoids too poor mire tracts with Sphagnum mosses and Eriophorum vaginatum, as well as parts with too deep water, too dense and high bushes or reeds, or too high sedge tussocks.

3.      Calcareous marshes with Cladium mariscus (Chelm marshes, Poland).

4.      Seasonally flooded brackish marshes of the Baltic Sea coast characterised by very weak and low reed stands 80–120 cm high in summer (in Germany, Swina river mouth in Poland, along the Curonian lagoon – Nemunas/Neman river delta in Lithuania).

5.      Wet marshy grasslands covered by high grass and clumps of sedge (in Hungary and in the Narew valley in Poland).

6.      Wet meadows of Phalaris arundinacea and Alopecurus pratensis cut once or twice a year, with sedge patches mainly of Carex gracilis, C. nigra, and C. disticha (Narew valley and lower Odra / Warta floodplains in Germany/Poland, Nemunas Delta in Lithuania).

During migration Aquatic warblers strongly favour low stands of sedges and reeds near open water, normally along rivers, estuaries and coastal lagoons (de By 1990).

The wintering habitats in the Senegal delta and Mali consist of large open grass marshes of Scirpus maritimus, S. littoralis and Sporobulus robustus. The Aquatic warbler occurrence is restricted to water-logged areas. Wintering Aquatic warblers were absent in dry grass marshes and such with scattered bushes and trees, in narrow Scirpus belts at lake shores, in deep water and half-open habitats, and especially in the vast high cattail Typha australis stands of the Diama reservoir (Flade et al. in prep.).

Life style

At first glance it seems like the aquatic warbler is an ordinary brown bird, but its lifestyle is incredibly interesting!

• Devoted mother. Females of Aquatic warblers breed twice per season. All summer they are growing their children – making nest, looking for food and protecting their chicks.
• Father Don Juan. Males of Aquatic warblers are taking care of their territory protection and starting new relationships. All summer they are looking for new romances. This is the strategy of species survival.
• Who is my dad? Since males and females have relationships with different partners, usually chicks in one nest could have different fathers.
• Sexuality. Males have gigantic testicles. It is as if human testicles were a size of large watermelon. Maximum recorded mating time – 45 min (while it is normally only a few seconds for similar birds).

 

Life Cycle

Aquatic warblers spend the biggest part of the year in wintering sites (September-March). In breeding sites, birds can be found only for 4 months (May-August). This short period is very important for aquatic warblers since juveniles are being raised. There are some dates to remember.

DatE	What are they up to?
May 1	return to breeding grounds
May 1-10	males sing and look for mates
May 10-20	females breed for the first time in the season
June 10-30	the first brood is able to escape mowing
July 1-15 d.	females breed for the second time in the season
August 15	the second brood is able to escape mowing
August 10-31	leave to migrate to wintering sites

The Aquatic warbler arrive at the Central-European breeding grounds in early May (exceptionally in late April), in W-Siberia probably not before mid-May. The first broods are started in mid-May, a part of the females perform a second brood starting in mid or late June. The latest young are fledged in mid-August.

Autumn migration starts earliest in July and is firstly directed to the West (Germany, Benelux, UK, France, Spain). Maximum numbers at the W-European stopover sites (Belgium, France) occur in mid-August, latest birds were observed in France and Spain in late September (Juillard et al. 2006, Atienza et al. 2001). The Aquatic warbler then go south along the SW-European and NW-African coast lines, using coastal wetlands as stopovers. The first birds arrive in North-Africa (Morocco, Algeria, Tunisia) in September and in West-Africa (West Sahara, Mauritania) in October (Schaffer et al. 2006).

At the wintering site in the Senegal delta, Mali first birds arrive in November (probably also late October) and stay there latest until mid-March. On the way back to the breeding grounds, first birds reach the North-African coast in February, most birds pass there in March and April (Schäffer et al. 2006). Several records from North-Italy, Switzerland and SW-Germany indicate that spring migration is performed a little bit more to the east and more on the direct way to the breeding grounds.

Home

Only a few places like these are left in Europe…

 

Aquatic warbler lives in this special habitat and is known as an “umbrella species”. This means that protecting the Aquatic Warbler will indirectly protect many of the other species living in the same habitat, like plants (like Orchids) and animals (like Black Tailed Godwits).

Food

Mainly insects: flies, ants and bees, butterflies, grasshoppers, dragonflies and spiders…

The life of babies

Many interesting things about life of Aquatic Warblers were observed when growing up translocated chicks in captivity.

Breeding

The Aquatic warbler has an extraordinary breeding system (review see Schulze-Hagen et al. 1999) with uniparental care by the female and a mating system between promiscuity and polygyny characterised by intense sperm competition. Around 59 % of broods are fathered by more than one male. Males, emancipated of almost all parental duties, sing and advertise throughout the whole reproductive season between early May and late July. Their home ranges average c. 8 ha, have a core area of c. 1 ha and overlap widely. Nests are built on the ground under dry sedges. Nesting aggregations can be found at sites of high productivity of arthropods. Nestling growth is owing to the uniparental care, retarded, nestlings fledge within 15-16 days. Nesting success is mostly very high, up to 83 %. Losses by predators make up ca. 11 % of nests, mainly by harriers Circus spec. and small mammals. Up to 50 % of females rear a second brood (Dyrcz 1993; Dyrcz & Zdunek 1993; Schulze-Hagen et al. 1993, 1995; Kozulin & Flade 1999; Kozulin et al. 1999).

Feeding

The diet comprises mostly large arthropods of the fen mires. Arachnida, Diptera, Lepidoptera (often caterpillars), and Trichoptera form about 70 % of prey. Prey composition varies enormously due to seasonal and annual fluctuation of the arthropod fauna. Compared with other Acrocephalus species, the nestlings are fed with relatively large insects (Schulze-Hagen et al. 1989). A rich supply of arthropods in the vegetation seems to be a prerequisite for the Aquatic warbler (Dyrcz & Czeraszkiewicz 1992).

Publications & videos

Join the journey of Aquatic Warbler babies

22 video stories from the second year of the first translocation of the species ever. Trip from Belarus to Lithuania in 2019.