Aquatic Warbler peripheral populations dynamics and its possible reasons in Lithuania

Abstract

Aquatic Warbler long time was mysterious bird in Lithuania. Despite some data about its breeding since the middle of 20th century and following some numbers from different sites over the country, only in 1995 for the first time was evaluated breeding population size - around 400 singing males. Since 2000, regular yearly population estimates (with few exceptions) for the country were calculated. The remaining northernmost breeding population of the species situated in 3 sites: Zuvintas, Nemunasdelta and coastal meadows alongCuronian lagoon. No other new sites of evident breeding during the last 14 years were detected. The number of singing males decreased gradually from 240 in 2000 down to 50 in 2013. The population decreasing in all of the sites. Zuvintas population with solitary birds is at the edge of extinction. Aquatic Warbler in Lithuania occupies habitats from seasonally flooded rich in nutrients Nemunas delta meadows to brackish Curonian lagoon coastal sedge meadows and sedge-dominated fens in Zuvintas. Management regime varies a lot among the sites, from totally abandoned to intensively using for hay making or grazing. Present abandoned and overgrown by reeds and bushes sites were used for hay making before 1990s. The decrease of population can be explained by two main reasons. First of all, abandonment of large formerly managed meadows, especially in Tyrai and Zuvintas, caused very big decrease of suitable habitat area. Secondly, very intensive management in Nemunas delta polder meadows during the last decades caused breeding success nearly to zero level. For short living bird, as Aquatic Warbler is, several years without descendants and with no immigration, leads to population crash