MEIJER R (2007) Disappearance of the Corncrake Crex crex from the Biesbosch. LIMOSA 80 (3): 89-95.

Until the beginning of the 1970s, the Biesbosch, a freshwater tidal wetland in the SW-Netherlands, was a hotspot for breeding Corncrakes. The birds were breeding in the less intensively used, semi-natural hayfields in the north-eastern part of what now has been designated as a national park. About 10-15% of the Dutch population occurred in the area and management of nature reserves was adapted to meet the habitat requirements of Corncrakes (delay of mowing). Since the early 1960s, the population has been monitored annually by nocturnal censuses. Different methods have been used but original data have been reanalysed for use in this paper. In the early 1970s all territorial males were ringed to study dispersion and philopatry. From the 1960s onwards, Corncrakes experienced a steep decline in the Biesbosch, from 40-100 singing males to 0-5 only ten years later. This decline had already startedmuch earlier and only the last part is shown by Fig. 1. Apart from a small and short upsurge around 1990, numbers have not recovered until today, and inmany years the species is absent. Moreover, the birds nowadays appear very late in the breeding season, mostly not before July, whereas formerly most territories were already occupied in May (Fig. 2).
      Part of the decline has been caused by habitat loss due to changes in land use (recreation, industry, switch from hay-making to arable land), intensification of agricultural practice and – recently – also grazing with low densities of livestock. As the species also disappeared from areas where such changes did not occur, there must be other reasons for the observed decline. Ringedmales showed a low philopatry: only three out of 29 individuals returned to the breeding grounds in the next year, and they were not recaptured in nearby areas, where Corncrakes were systematically ringed as well. This implies that the population in the Biesbosch was part of amuch wider population and local numbers were also heavily influenced by declining numbers elsewhere. Strikingly, the recovery of the species in the Netherlands after 1997,when all former breeding sites were occupied again (also along the nearby RiverWaal, Fig. 1), was not observed in the Biesbosch at all. Apparently local habitat conditions are no longer suitable for Corncrakes. It is proposed that closure of the Haringvlietsluizen in 1970 and subsequent reduction of tidal amplitude is themain factor causing this. As a result, deposition of fertile silt by natural inundations became infrequent, affecting vegetation growth and structure. Instead, chemical fertilizers were increasingly applied by farmers, causing a uniform and dense vegetation that is unattractive to breeding Corncrakes. In those parts of the area designated as a nature reserve, reduced nutrient input caused a delayed vegetation growth and a more open vegetation structure with little cover for Corncrakes upon arrival inMay. It is not expected that the specieswill return to the Biesbosch in decent numbers under the current conditions.

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