• Summary
• Text account
• Data table and detailed info
• Distribution map
• Reference and further resources

Justification of Red List category
The population is restricted to a small core breeding area within Europe, with 99% population found at sites in Poland, Belarus and Ukraine. A significant proportion of the breeding habitat could be rapidly affected by droughts, fires, or floods, and the species is assessed as having fewer than 10 locations for these threats. A population decline is ongoing due to habitat degradation, even in areas subject to extensive conservation efforts; outside the core range, rapid declines continue and further national extinctions are also likely. The small breeding range, few locations and continuing decline mean that Aquatic Warbler is assessed as Vulnerable.

Population justification

In Europe, the breeding population is estimated to number 9,100-14,300 calling males, which equates to approximately 18,000-29,000 mature individuals and 27,000-43,000 individuals (BirdLife International 2021). The entire breeding population is contained within Europe, with 99% of the population found in Poland, Belarus and Ukraine (the Central European subpopulation), so an estimate of the global population size is 18,000-29,000 mature individuals. Furthermore, previous research from Tanneberger and Kubacka (2018) estimated the global population to be less than 11,000 singing males, while Briedis and Keišs (2016) provided a range of 11,000-16,000 singing males, or 22,000-32,000 mature individuals.

Trend justification
The west Siberian population in European Russia is probably extinct, with the last confirmed record in 2000 (Tanneberger and Kubacka 2018). The Hungarian subpopulation collapsed from c. 700 singing males to 331 in 2002, with another collapse in 2006, and a final collapse in 2012 – no singing males have been observed since (Végvári and Flade 2012). There are varying rates of decline across the global range, less than previously observed. While it appears that the large population in Poland is stable or even increasing, the Belarus population declined by 47-55% between 2000-2011 (BirdLife International 2015) and the Pomeranian population has declined to critical levels. The Lithuanian population appears to have recovered since 2013 to 247 s.m. in 2016 (Ž. Morkv?nas in litt. 2016).
In Europe, the population size is estimated to be decreasing by approximately 15% in 10 years (Birdlife International 2021). While the global population appears stable, some subpopulations are declining (Tanneberger and Kubacka 2018) as well as extinction of small peripheral populations (Zmihorski et al. 2016). Fluctuations from large breeding sites, as well as the weakness of counting methods, may also be masking a slow decline. Harmful impacts from pollutants may be an emerging issue, but its impact on the population has not been investigated (Pacyna et al. 2018).
As the majority of the breeding population is contained within Europe, and in the face of possible local extinctions, the global population is inferred to be declining at a rate of 10-19% over 10 years, primarily due to severe habitat degradation.

This species breeds across a scattered range at fewer than 60 regular breeding sites in the following countries, with numbers given of singing males reported between the years of 2007-2016/17 (Tanneberger and Kubacka 2018): Poland, 3,024; Belarus, 4,120; Ukraine, 3,653; Germany, 0-2 (absent since 2014); Lithuania, 110; and Western Siberia, 0-500. 

The breeding population has been divided into a central European core subpopulation, including Belarus, eastern Poland and Northwest-Ukraine; a central Ukrainian subpopulation, east of the Dnipro river; a Lithuanian subpopulation (likely to be connected with the Central European population through small subpopulations in southeastern Lithuania and northern Belarus); an isolated Pomeranian subpopulation in Northwest-Poland and Germany; a west Siberian population; and a Hungarian population (Tanneberger and Kubacka 2018). The last appears now to be extinct, and there is uncertainty over the viability of the west Siberian population.

Post-nuptial migration initially occurs in a westerly and south-westerly direction with birds passing both north and south of the Alps (Jiguet et al. 2011, Salewski et al. 2013) and it has been recorded from all European countries except Malta and Cyprus. The initial key stopover sites appear to be in west and southwest France (Jiguet et al. 2011, Latraube and Le Nevé 2014, Marquet et al. 2014) and the northern Iberian Peninsula (Salewski et al. 2013). The species winters in the Sahelian belt of sub-Saharan West Africa (Schäffer et al. 2006, Walther et al. 2007). Wetlands along the lower Senegal River, especially those within and to the north of Djoudj National Park are significant wintering areas (Bargain et al. 2008, Flade 2008, U. Malashevich in litt. 2012, Aquatic Warbler Conservation Team 2021), as are wetlands in south-west Mauritania and the inner Niger Delta in Mali (U. Malashevich in litt. 2012, Schäffer et al. 2006, Walther et al. 2007, Aquatic Warbler Conservation Team 2021).

It breeds in large open lowland sedge fen mires and similarly structured marshy habitats with a water depth of 1-10 cm (Aquatic Warbler Conservation Team 1999, BirdLife International 2008). In strongly euthrophic habitat the species is more or less dependent on human management to maintain open habitat, while in poorer areas water table variation may be sufficient to maintain suitability; however the species avoids very poor mires (BirdLife International 2008). Targeted agri-environment schemes can maintain suitable breeding habitat for the species in central Europe, especially where conservation action has already taken place (BirdLife International 2008). The highest densities of singing males were found where vegetation was between 60 and 90 cm high and densities increased with percentage of ground covered by water (Kloskowski et al. 2015). In some coastal areas, the species will nest in the sedge-like salt marsh-grass-sea spurrey plant community (Dyrcz et al. 2020).
On migration the species uses low stands of sedges and reeds, seemingly the latter for resting and the former for feeding (Bargain 2002). The sites where it is found during migration are mostly coastal marshes, lagoons and estuaries, but there are also regular records at sites along large rivers (de By 1990). It winters in the grassy saline Scirpus, Eleocharis and Oryza marshes of the Senegal and Niger deltas (Flade et al. 2011), but will avoid dense homogeneous areas dominated by S. maritimus (Dyrcz et al. 2020).
It has an altitudinal range of 0-653 m (Carbajo et al. 2019). According to a study conducted in the wetlands of northwestern Spain, the species largely feeds on small arthropods (Miguélez et al. 2016), these include spiders, dragonflies, grasshoppers, earwigs, water bugs, aphids, neuropterans, lepidopterans, caddis flies, dipteran flies, hymenopterans, beetles, and water snails (Dyrcz et al. 2020).

The most important threats are loss of breeding habitat owing to drainage for agriculture and peat extraction, damming of floodplains, unfavourable water management and the canalisation of rivers. Habitat degradation is widespread where traditional fen management has ceased, allowing succession to unsuitable overgrown reedbed, scrub or woodland (Aquatic Warbler Conservation Team 1999, BirdLife International 2008). Any changes that alter the hydrological regime of breeding sites result in reduced breeding success and population declines (BirdLife International 2008).
Uncontrolled fires in spring and summer pose a direct threat to birds and nests, and can burn out the upper peat layer of fens (Aquatic Warbler Conservation Team 1999, BirdLife International 2008), although in certain locations controlled burning on frozen ground in winter may increase the density of singing males in following seasons (Grzywaczewski et al. 2014). Similarly, grazing at too high an intensity, cutting too early or too frequently may cause significant local reductions in breeding success (BirdLife International 2008, Ž. Morkv?nas in litt. 2016).
In the wintering grounds, agricultural cultivation and irrigation (creation of rice and sugar cane plantations), drought, wetland drainage, intensive grazing, succession to scrub, desertification and salinisation of irrigated soils are all potential threats (Aquatic Warbler Conservation Team 1999, M. Flade and L. Lachmann in litt. 2007). Lastly, there may be an emerging threat of mercury contamination, especially during the breeding season in Europe, however this requires more investigation (Pacyna et al. 2018). 

Conservation Actions Underway
CMS Appendix I and II. EU Birds Directive Annex I. It is legally protected in all countries of its breeding range except Ukraine and Russia (U. Malashevich in litt. 2012). All key breeding sites in Belarus, Germany, Hungary and Poland are located within protected areas (Aquatic Warbler Conservation Team 1999, U. Malashevich in litt. 2012). In Ukraine, 85% of breeding habitat is within protected areas (Poluda 2015). Habitat is actively managed in Poland, Belarus, Ukraine, Lithuania, Hungary and Germany, whilst across the EU, conservation and EU LIFE projects are in place with particular focus on habitat restoration, establishment of farming/compensation schemes, and promoting alternative use of late cut biomass (Flade 2015, Ž. Morkv?nas in litt. 2016). All breeding range states but Russia have monitoring programmes (Aquatic Warbler Conservation Team 1999, M. Flade and L. Lachmann in litt. 2007). A study to identify the wintering range of the species was conducted in 2007 (Flade 2008). Studies on halting succession have been conducted in Belarus, Poland and Ukraine (M. Kalyakin in litt. 1999). A European action plan was published in 1996 (Heredia et al. 1996) and updated in 1998, 2003, 2008 and 2015 (Flade and Lachmann 2008, BirdLife International 2008). The species was put into agenda of the Conference of Parties for the CMS in November 2011, where a special resolution on African-Eurasian landbirds was adopted, which applies to Aquatic Warbler (U. Malashevich in litt. 2011). There are plans to perform a conservation translocation in Lithuania (Ž. Morkv?nas in litt. 2016). There are a number of existing awareness activities across Lithuania, targeted at farmers, children, and decision makers (Ž. Morkv?nas in litt. 2016).

Conservation Actions Proposed
Establish if the species continues to persist in west Siberia. Investigate the potential threat of mercury contamination. Research alternative solutions for the use of biomass from fen mires and develop sustainable use of biomass in the long-term. Develop a management plan. Implement permanent ringing stations and greater field excursions to identify the species at stopover sites.Protect key breeding sites. Support population connectivity and metapopulation dynamics with habitat restoration and protection. Increase reintroduction programmes to restored habitats. Promote protection of the species and its habitat in wintering areas and along the migration route (including conservation of unknown staging sites in the northern Mediterranean, Iberian Peninsula and northwest Africa; Salewski et al. 2019). Ensure full legal protection.

12-13 cm. Small, heavily streaked, buff-and-black warbler. Strong black streaking on mantle bordered by pale "tramlines". Pale coronal stripe with black border. Pale lores. Streaked back, rump and uppertail-coverts. Finely streaked breast. Similar spp. Sedge Warbler A. schoenobaenus generally browner and less streaked. Lacks strongly streaked mantle and pale "tramlines", pale coronal stripe and streaks on breast. Song more complex and varied with mimicry and distinctive sweet notes. Voice Series of simple trills and short whistles, given from vegetation or short song flight. Does not mimic extensively. Low tuk or dry churr call.

Text account compilers
Fernando, E., Martin, R., McGonigle, K.

Contributors
Ashpole, J, Burfield, I., Capper, D., Derhé, M., Ekstrom, J., Flade, M., Grice, H., Kalyakin, M., Khwaja, N., Krogulec, J., Lachmann, L., Malashevich, U., Morkv?nas, Ž., Peet, N., Piggott, A., Rutherford, C.A., Staneva, A. & Symes, A.

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Recommended citation
BirdLife International (2024) Species factsheet: Aquatic Warbler Acrocephalus paludicola. Downloaded from https://datazone.birdlife.org/species/factsheet/aquatic-warbler-acrocephalus-paludicola on 21/12/2024.
Recommended citation for factsheets for more than one species: BirdLife International (2024) IUCN Red List for birds. Downloaded from https://datazone.birdlife.org/species/search on 21/12/2024.