VOSKAMP P & ZOETEBIER D (1999) Breeding of Night-heron Nycticorax nycticorax in the forelands of the river IJssel: territories, nests, habitat and behaviour. LIMOSA 72 (4): 133-142.

A small breeding population of Night Heron was studied in the forelands of the river IJssel (part of the Rhine Valley). The study area (2200 ha, Fig. 1) comprises a varied riverine landscape of farmland as well as nature reserves. Here, riverine forests (60 ha) and marshes (37 ha) dominate. The occurrence of Night Herons in the breeding season has been reported from 1975 onwards (Tab. 1). Territories and nests were found in only two areas, whereas more than two territories were never recorded in one single year. Nests where found in 1986, 1987 and 1996. All nest-sites (Tab. 2) were located in riverine woodlands (mainly Salix) with rich undergrowth and situated in colonies of Grey Herons and Cormorants. Breeding started on average around the 15th of May (N=4). The number of fledglings per successful nest averaged 2.5 (N=4). Night Herons in this area proved to be very silent during the breeding season, probably as a result of their occurrence in solitary pairs. When foraging, the birds appeared to be extremely shy and very sensitive to disturbance by humans, with disturbance distances of up to 500 meters. As a result, the birds were feeding significantly more at sites with cover during daytime (Tab. 3), and only went foraging at distant sites up to 4 kID at dusk. Median length of foraging flights measured 0.9 kID during day and 1.7 kID in the evening (Fig. 2). Foraging at daytime only occurred during the period when there where nestlings to be fed (Fig. 3). Night Herons probably are regular breeders in the area. Breeding attempts however are easily missed by observers, due to the secretive and nocturnal habits of the species. The carrying capacity of the area is in the present situation pro- 140 bably not much higher than the maximum of two pairs found. It is discussed that modem ecological restoration in the river area, in which extraction of minerals is combined with the creation of new wetlands, will generate good quality habitat for Night Herons. This expected growth in carrying capacity will probably only be beneficial for Night Herons when other preconditions such as good water quality and minimal human disturbance are also met.

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