ERHART FC & KURSTJENS G (2000) Black-crowned Night Herons Nycticorax nycticorax as a breeding bird in the Netherlands in the 20th century. LIMOSA 73 (2): 41-52.

The history of the Dutch Night Heron breeding population in the twentieth century is reconstructed . Breeding cases of this species are very hard to find in The Netherlands, mainly as a result of the secretive behaviour of the (solitary) breeding pairs. Moreover, bird mapping methods currently used in the Netherlands (based on mapping of territories) are not always suitable for detecting breeding Night Herons as the birds lack any sign of pronounced territorial behaviour. Therefore, we used a cumulative method of estimates per decade for the reconstruction, making use of certain and probable breeding cases in which nests or adults with young were observed, as well as possible cases, which were defined by two or more records in suitable breeding habitat between 1 May-15 July.

The development of the breeding numbers has been subject to substantial fluctuations (Fig. 2, Tab. 2). After World War 11, Night Herons were found breeding at four sites, including the Biesbosch and an area near the Groote Peel, altogether with an estimated 40-60 pairs. In the 1980s, less than 5 certain breeding cases were found while the estimated number of breeding pairs had dropped to a level between 10 and 22. Strikingly, the 1990s showed a light increase up to 12-35 cases, distributed over 26 sites. This increase is connected with the release of at least 80 birds from zoos at different sites in The Netherlands from 1987 onwards (Tab. 1). In 1999, the cities of Amsterdam and Rotterdam accommodated 10 and 2 free-living pairs of Night Heron respectively. Since 1978, released birds have been reported to nest in the reserve Het Zwin, just across the border in Flanders, Belgium. Breeding of these birds has been stimulated by feeding in winter, which has resulted in overwintering of (adult) birds in the surroundings of the zoos and bird parks. At least part of the birds (fledged young) migrates to the south (based on ringing recoveries in France, Sicily and Algeria), as is the case with released White Storks in The Netherlands. In addition, the number of winter observations of Night Herons has increased considerably in comparison with 15 years ago (Fig. 1). The decrease of the breeding numbers has often been attributed to loss of breeding habitat, lack of food, deteriorating water quality, and locally also by disturbance due to increased recreation. The recent and local increase of possible breeding cases might be explained by the northward expansion of the breeding populations in e.g. France. A similar pattern has become obvious in Little Egret in the past decade. In addition, ecological restoration projects in the floodplains of the Rhine and Meuse rivers have improved breeding conditions of Night Heron. The projects include restoration of the former dynamics of the river and its side-channels, leading to enhanced feeding opportunities (Iargerpopulations of fish and amphibians) and breeding habitat (development of alluvial forest). Similar projects are carried out or planned for peat-bog areas. On the other hand, it remains unclear whether settlement of Night Herons is restricted by recreational use and other sources of human disturbance. Compared to the past decades however, overall breeding conditions have increased considerably and we therefore expect the breeding population of Night Heron in The Netherlands to grow in the years to come. Black-crowned Night Heron Nycticorax nycticorax

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