Romke Kleefstra (2025) From reed vegetation into Sea Buckthorn thickets: habitat switch of Western Marsh Harrier Circus aeruginosus on the Dutch Wadden Sea island of Schiermonnikoog. LIMOSA 98 (2): 59-66.

Nests of Western Marsh Harriers were studied from 2006 up to 2024 in four breeding bird monitoring plots on the salt marsh Oosterkwelder on the Dutch Wadden Sea island of Schiermonnikoog (Fig. 1). Because the nest survey had to be carried out during the mapping of breeding birds, not all nests could be found due to a lack of time. However, it was possible to determine the breeding habitat for all nests based on nest-indicative behaviour and nest finds. Until 2012, all nests were located in reed vegetation, but from then on more and more pairs settled in and among Sea Buckthorns Hippophae rhamnoides. In 2023 and 2024 25 of a total of 26 pairs nested in these shrubs (Fig. 3). The area dominated by Reed was less than 5 ha in 2005, but expanded to approximately 20 ha in recent years. The area of Sea Buckthorn was approximately 27 ha in 2005, almost exclusively in the western part of the study area. Expansion of Sea Buckthorn scrub throughout the study area (Fig. 1) resulted in a coverage of over 64 ha in 2012, 75 ha in 2016 and approximately 170 ha in 2022. It is unclear why the harriers make the habitat switch to a rapidly increasing cover of Sea Buckthorn thickets. It is not because reed vegetation decreases in area, because it also expanded, nor because of higher breeding success in Sea Buckthorn: instead more nests are successful in reeds than in Sea Buckthorn (Tab. 1). Increased predation pressure also does not seem to be the cause. Potential ground predators present include Hedgehog Erinaceus europaeus, Brown Rat Rattus norvegicus and feral House Cat Felis silvestris catus, but predation of none of these three was recorded in the period 2006-24. A conceivable reason could be that by breeding the Sea Buckthorn thicket, Marsh Harriers avoid the widely spread and numerous large breeding gulls such as Lesser Blackbacked Gull Larus fuscus and Herring Gull L. argentatus as direct neighbours, which is virtually impossible in the reeds. Harriers are frequently attacked when flying to and from nests in reeds, which is not the case if the harriers nest in the middle of Sea Buckthorn thickets. However, this is only a hypothesis. A comparison with the habitat choice of harriers in the Netherlands based on 5023 nests in the period 1984-2023 indicates that probably less than 1% of these Dutch pairs breed in/between Sea Buckthorns. The habitat switch on Schiermonnikoog is therefore all the more special.

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