STRUCKER R.C.W. (2020) Changes in breeding bird numbers and habitats in the tidal freshwater zone along the river Oude Maas. LIMOSA 93 (2): 59-73.

After closing off the Haringvliet inlet in 1970 the Oude Maas became the most important remaining tidal freshwater zone with a considerable tidal amplitude (on average 1.1 m) in the Netherlands. A number of nature reserves with a total area of 576 ha is situated along the shores (Fig. 1).
Due to a wide variety of vegetation types, tidal freshwater zones are characterized by a large diversity of breeding birds. During surveys in 2010-19 a total of 84 breeding bird species were recorded. In comparison with 1985-1994 (total of 72 species), 20 new species settled and eight species disappeared (Tab. 1). Waterbirds generally showed increases and several new species (e.g. Greater Canada Goose and Greylag Goose) settled in the area. Woodland birds showed positive trends as well, while the picture was less clear for birds from reedbeds. Species like Sedge Warbler, Reed Bunting and Savi’s Warbler increased, but other species decreased (Moorhen) or disappeared as an annual breeding bird (Great Reed Warbler). The majority of the breeding birds of shrubs and meadows showed negative trends, in accordance with national trends. Notable exceptions were Cetti’s Warbler (strong increase), Redshank and Meadow Pipit (increase).
Most breeding birds had a clear preference for a specific habitat type (Tab. 2). The largest number of species (60) occurred in overgrown reedbeds. This also included shrub birds such as Marsh Warbler, Cetti's Warbler and Bluethroat. The second highest number of breeding bird species was found in willow woods (56), followed by the regularly cut osier beds (41), rows of trees (32), meadows (31) and regularly mowed reedbeds (20). The highest density of breeding birds (all species combined) was found in willow woods (11.7 pairs/ ha).
A key aspect for the future trends of breeding birds is habitat management. As habitats are no longer commercially managed (harvest of reed and willows has halted), natural succession results in reedbeds getting overgrown with tall forbs, and willow shrubs and osier beds developing into willow woods. If this management does not change, the typical variation in habitats will disappear, which will have considerable negative consequences for the variety of breeding birds. Restoring the historical management actions is essential to preserve the rich breeding bird diversity of the Oude Maas tidal freshwater zone.

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