Effect of mowing on productivity in the endangered Aquatic Warbler Acrocephalus paludicola

Abstract

SummaryThe Aquatic Warbler Acrocephalus paludicola is a globally threatened habitat specialist that breeds in open fens in Central and Eastern Europe. Because bush and reed encroachment threaten many suitable breeding areas, habitat management is necessary to maintain the open wetlands that Aquatic Warblers require for nesting. The effectiveness of mowing as habitat management has so far only been assessed by counting the number of singing males. To assess whether mowing also affected vital reproduction parameters, we analysed Aquatic Warbler productivity in the Biebrza National Park, Poland, on plots in four different successional stages after mowing. Our study showed that productivity was lowest in the first year after mowing, but increased to the highest levels in the second year after mowing. The productivity differences between areas at different stages after mowing resulted from differences in nest density, since we found little evidence for an effect of mowing on nest survival or the number of fledglings produced per successful nest. Nest survival was highly variable between years and varied mostly with nest age and nest initiation date. The density of singing males was positively correlated with both the nest density and the number of fledglings produced in an area, suggesting that this simple indicator could be used to rank the quality of Aquatic Warbler habitats. We recommend that in mesotrophic fen mires, such as the Biebrza valley, mowing as habitat management is applied less frequently than every second year.

Water on the Fen Mire as a Problem in the Protection of Globally Threatened Species: Long-Term Changes in Aquatic Warbler Numbers

“…After this area was mown in autumn 2011, there was a distinct rise in the number of singing AW males in the following season. Similar effects were observed on the Biebrza [11,37] and in Polesie National Park [20].…”

“…The density of singing AW males is a good indicator of the density of females as the higher density of males also means a higher density of females. But densities are influenced by water level because when the water level is too high (above 20 cm), females do not build nests [3,7,11]. Water affects the accessibility of nests to predators, the composition and abundance of food resources, and, hence, breeding success [3,7,11].…”

“…But densities are influenced by water level because when the water level is too high (above 20 cm), females do not build nests [3,7,11]. Water affects the accessibility of nests to predators, the composition and abundance of food resources, and, hence, breeding success [3,7,11]. Although AW has been the object of a many investigations during the last 20 years [7,[11][12][13][14][15][16][17][18][19][20], the relevant literature does not boast a set of long-term data gathered every year on the same area, especially in the context of water-level changes.…”

Song Post Selection in the Aquatic WarblerAcrocephalus paludicola

“…When population size in a given site and year was reported as a range we used average values. In general, at most of these sites regular counts of singing males were performed, which is known to reflect the number of breeding females (Dyrcz & Zdunek 1993, Kubacka et al 2014). Singing males were counted twice per season, during their highest activity periods in their first and second breeding attempt (20 May-10 June and 20 June-10 July).…”

Effect of Habitat Burning on the Number of Singing Males of the Aquatic WarblerAcrocephalus paludicola

“…One of the species strongly dependent on extensively managed marsh habitats is the Aquatic Warbler Acrocephalus paludicola, a globally threatened passerine with a world population estimated at 10,200-14,200 singing males (Flade & Lachmann 2008). The major reason for this decline has been the loss of breeding habitat (Flade & Lachmann 2008, Kubacka et al 2014. The Aquatic Warbler is a habitat specialist, breeding in broad lowland river valleys, mainly on mesotrophic and slightly eutrophic sedge fen mires, with water depth of 1-10 cm (Flade & Lachmann 2008.…”