VAN DER WINDEN J, DEUZEMAN S, WEEDA S, FOPPEN R, VAN HORSSEN P & POOT M (2020) Breeding success and nesting habitat of the Great Reed Warbler Acrocephalus arundinaceus in deteriorated reeds. LIMOSA 93 (4): 153-164.

Marsh bird populations in the Netherlands seem to have recovered recently, except for the Great Reed Warbler. Numbers in the Netherlands have declined from 5000 territories in the fifties to less than 100 today. Deterioration of water reed is currently the most important factor for this decline, caused by overgrazing by Greylag Geese Anser anser. From 2013 onwards a number of measures (mainly fencing of reedbeds) have been taken in Great Reed Warbler core areas to prevent the geese from grazing down reed fringes bordering the lakes. Although this resulted in an increase of reed quantity, an increase in Great Reed Warbler numbers has not been observed yet. This period may be too short to measure changes in population size, but possibly other factors currently also affect Great Reed Warbler's reproduction and settlement. In a three-year study we measured breeding success and nesting habitat selection of Great Reed Warblers in two core areas: Loosdrechtse Plassen and the northern Randmeren. In 2017 we weekly monitored pairs and checked from a distance whether they were nesting and raised young. Since 2018, we decided to check nests at a weekly basis to obtain a more detailed picture (N=34 in 2018 and N=54 in 2019). Clutch size was similar between the two core areas, however average breeding success varied: in the Randmeren parents on average raised 2.4 young, while in Loosdrechtse Plassen this was only 1.3 fledged young per pair. The difference in breeding success between the two core areas was mainly caused by nest predation. Incidental records indicated Brown Rat Rattus norvegicus as predator. Also the breeding success for successful nests (nests that resulted in at least one young) was lower for the Loosdrechtse Plassen (2.4 fledged young per pair) compared to the Randmeren (3.7 fledged young per pair). This difference suggests that food availability might be a limiting factor in the Loosdrechtse Plassen. We found that Great Reed Warblers in the two core areas nested preferably in broad reed fringes in deeper water, away from both the water edge and the land. Furthermore, nest success was significantly lower in reed fringes less than 10 m wide. We conclude that the deterioration of reed beds by overgrazing by geese enhances predation and potentially decreases food availability (inferior habitat quality). If grazing by geese continues it will ultimately result in unsuitable breeding habitat for Great Reed Warblers. Therefore, it is critical to proceed with measures to enlarge and restore reed beds. In the years to come, we will continue monitoring the impact of reed fencing on the breeding success of Great Reed Warblers.

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