Structural effects of reedbed grazing and its cessation on reed-nesting songbird densities

Abstract

Reedbeds are crucial breeding habitats for vulnerable songbird species. Irrespective of their protection status, these habitats may be threatened by organic matter accumulation, progressively leading to structural homogenization and habitat succession towards woodland. Managers prevent excessive litter build-up with various interventions opening-up the reedbeds, such as grazing, which may be detrimental or suitable for some reed bird species. We assessed the effects of extensive grazing by horses and its cessation in the medium and long terms on reedbed structure, and the consequences on reed-nesting songbird densities in Estagnol Nature Reserve, a protected wetland near the French Mediterranean coast. We compared reedbed structural features between grazed, newly ungrazed and old-ungrazed plots. During nine years, we censused four songbird species in spring and collected water level data in the same survey plots. Grazing reduced reedbed extent, rejuvenated the vegetation with more short green stems on a thinner litter, and produced higher structural heterogeneity and discontinuity compared to long-lasting non grazing. Newly ungrazed plot showed intermediate effects. All surveyed songbird total densities were similar among plots while species numbers and densities differed. Grazed reedbed was more attractive to Moustached Warblers (Acrocephalus melanopogon) and Great Reed Warblers (Acrocephalus arundinaceus), likely due to the large edges and the high vegetation structural heterogeneity. However, Moustached Warblers were more negatively affected by higher water level in grazed reedbed, presumably because flooding prevents litter foraging. The newly ungrazed reedbed was not preferred by any species. All warbler species were found under low density in the old-ungrazed reedbed, where Reed Buntings (Emberiza schoeniclus) were exclusively found. Food availability related to thick litter layer may explain this predilection. Common Reed Warblers (Acrocephalus scirpaceus) were found everywhere in similar densities. We conclude that reedbed management by extensive horse grazing and also its long-term cessation may benefit several songbird species. Within a context of wetland degradation and disappearance, both management strategies are useful to establish and maintain suitable habitats for reed-nesting songbird communities.