Braaksma S. & Mörzer Bruijns M.F. (1954) De stand van de Roerdomp, Botaurus stellaris L., als broedvogel in Nederland tot 1953. ARDEA 42 (1-2): 151-162

An investigation concerning the distribution of the Bittern, Botaurus stellaris L., in the Netherlands has been conducted by the Nature Protection Department of the Government Forestry Service, with the aid of about 150 Dutch amateur ornithologists. This investigation proves that at least in 280 localities Bitterns have been breeding in the Netherlands during the season 1950-1951. The average number of annual breeding cases during this period was probably somewhat higher and can be estimated at about 300-330. The breeding results of 1950-1953 are taken as an average, because during this period the Bitterns had recovered from the losses during the severe winter of 1947. Bitterns prefer breeding in marshy areas with high vegetation of reeds and rushes. Especially they prefer such localities in which vegetation is only partially cut down during winter. Bitterns start their breeding at an early date chiefly in the month of April, when the fresh reeds are still too short to give sufficient covering to the breeding birds. Therefore mowing and burning of vegetation should be limited to part of the breeding area only, especially in nature reservations. Breeding localities in the Netherlands can be divided into ten different types, viz. large lowland marshes with about 40 %of the total number of breeding birds, shores of the large lakes with about 13 %, moor-fens or former moor-fens in heathlands, closing with reeds, due to eutrophication with about 7 %, brooks and cut-off courses of brooks with about 2 %, marshy areas in river valleys, such as river-shores, excavations by brickworks and duck-decoys, with about 15 %, dune lakes with about 3 %, marshes in the river deltas with about 7 %, shore-lands of the former Zuiderzee and North-East Polder, as far as not yet drained, with about 4 %, creeks, small lake- and marshy areas in polders with about 10 %, and peat-bogs without any records of recent breeding cases. About 15 to 20 %of the total number of breeding birds in the Netherlands is restricted to nature reservations and these will most probably maintain their position. For the rest the future of the Bitterns in the Netherlands is precarious. From about 30 localities, known as breeding grounds in the beginning of this century Bitterns have disappeared. Possibilities for new settlements are few, such as reed-lands in new polders, eutrophiating moor-fens, and shores of the new freshwater lakes in the area of the former Zuiderzee. On the other hand, many areas with favourable conditions are disappearing by draining of marshes, cleaning of moor fens, canalizing of rivers and cultivating of excavations by brickworks. Recreation on the large lakes is increasing, motorboats and landing of other vessels do great damage to reeds and rushes near the shores. Bitterns are completely protected by law in the Netherlands. Poachers and egg-collectors, however, seem especially keen on finding this species. The establishment of carefully protected breeding grounds in nature reservations seems to be the best way to ascertain the survival of the Bittern as a breeding species in the Netherlands.