MEIJER R & WEEL B (2007) Breeding birds of the Biesbosch National Park. LIMOSA 80 (4): 139-152.

Until 2 November 1970, the Biesbosch was a fresh water tidal area in the SW Netherlands. During almost 550 years it was shaped by the rivers Meuse and Rhine and by people who harvested osiers, reed and rushes. Parts of the area that had silted up high enough were dammed and changed into polders with grassland and arable land. Until 1970 most of the area was inundated twice a day, making it impossible for birds to nest near the ground. Almost all reed beds were cut every winter, limiting possibilities to breed for birds preferring old reed. As tall trees were rare, woodland birds like Blackcap Sylvia atricapilla and Great Spotted Woodpecker Dendrocopus major were scarce. As a consequence the Biesbosch was rather poor in both species and numbers of breeding birds. The specialities of those days were a colony of Black-crowned Night Herons Nycticorax nycticorax, 50-100 Corn Crakes Crex crex and many hundreds, maybe up to 1000, Great Reed Warblers Acrocephalus arundinaceus.
      When the Haringvliet sluices were closed in 1970, the tides almost disappeared in the south-western part (from c. 1.8 m to 0.3 m) and halved in the north-eastern part of the Biesbosch (from 1.4 m to 0.7 m). The reed beds dried out and within a few years changed into fields of nettles and other high herbs, while Willow Salix spp. and Elder Sambucus nigra established themselves. As transportation of osiers became impossible the exploitation of almost all holms came to an end and they changed into Willow woods. Three large polders were changed into drinking water basins (Fig. 1). To prevent flooding of the surrounding densely inhabited areas, a total of 2,100 ha of arable land is renaturated.
      All these changes had an enormous impact on breeding bird populations. The number of breeding species increased (Fig. 2, Table 1, 2) and a total of 154 species are now known to have bred in the area; almost two thirds of all species breeding in the Netherlands. The number of species breeding in any one year increased from 85 to 115. The total number of pairs/territories doubled (Fig. 3). Nowadays, woodland birds dominate the species top-10, whereas around 1970 the list was more mixed (Table 3). The consequences of the succession from holms to Willow woods are demonstrated for three species (Fig. 4): Great Spotted Woodpecker (strong increase followed by a sharp decline due to loss of many trees in a severe storm), Willow Tit Parus montanus (loss when holms disappeared, profit from dying trees and a sharp decline when dead trees were covered with a blanket of climbing herbs) and Blackcap (very strong increase). Many woodland birds show similar patterns. The main losers are the red-listed Icterine Warbler Hippolais icterina and Common Linnet Carduelis cannabina; both did better in the holms. European Turtle Dove Streptopelia turtur and Eurasian Golden Oriole Oriolus oriolus also declined, but the causes of these losses probably lie outside the area.
      The consequences of the change from reed beds to fields with high herbs are more mixed (Fig. 4). True reed breeders like the Common Reed Bunting Emberiza schoeniclus and Common Reed Warbler Acrocephalus scirpaceus sharply declined at first, but remained rather stable during the last 20 years. Great Reed Warbler nowis a rare species. Linnet increased thanks to the new shrubs of Willow and Elder, showed a very sharp decline after three severe winters in the early 1980s and never recovered as those shrubs grew too high. All birds preferring a vegetation of high herbs mixed with shrubs increased. Examples are the red-listed Common Nightingale Luscinia megarhynchos and Savi's Warbler Locustella luscinioides, and Common Grasshopper Warbler Locustella naevia.
      This paper only gives a necessarily condensed impression of the changes during the last decennia. Until the Haringvliet sluices will be partly opened again around 2015 the question remains what the Biesbosch would have looked like nowadays if the tides would have remained and the intensive exploitation would have ceased in 1970.

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