The Aquatic Warbler (Acrocephalus paludicola) is the rarest passerine bird species found in mainland Europe. With a very small world population, of around only 12,000 to 20,500 pairs, it is classified as vulnerable on the Red List of Endangered Species and is a UK BAP Priority Species. Once widespread and numerous, this habitat specialist has disappeared from most of its former range in northern Germany and Poland. The genetically distinct population along the German-Polish border is now acutely threatened and the core population in north-eastern Poland has been restricted to very few sites. The Aquatic Warbler is listed in Annex I of the Birds Directive, and is a priority species for European Community funding under the LIFE programme. Most range states, including Poland and Germany, have signed an international Memorandum of Understanding for the aquatic warbler, which commits them to implementing an International Action Plan (IAP) for this species and its habitat.
A minimum of 44 Aquatic Warblers have been reported at South Milton Ley since 1969, (Devon Birds annual reports), making it one of the top ten sites for the species in England. Almost all of the records result from birds being trapped during ringing sessions. Numbers at the reserve have declined in line with the national trend, (see below).
Whilst it is impractical to maintain areas within the reserve specifically for the few Aquatic Warblers, which visit the site on autumn passage, future management activities should consider the requirements of this species whenever possible.