BOER P DE & KLAASSEN O (2007) Reproduction and diet of Hen Harriers Circus cyaneus on the Wadden Sea islands of Terschelling and Ameland. LIMOSA 80 (4): 129-138.

The DutchWadden Sea islands represent the core breeding range for Hen Harrier in the Netherlands (>90%of the national breeding population) and the international Wadden Sea (80% of the population in the Dutch, German and DanishWadden Sea). After the first breeding record on Ameland in 1940, the population increased to 110-120 breeding pairs in the early 1990s with settlements on nearly all islands. However, the past decade has shown marked declines, especially on Terschelling and Ameland (Fig. 2). In order to investigate the backgrounds of this decline, studies were started on Terschelling and Ameland in 2004. Themain aimwas to assess the number of nests, breeding status of males and females (polygamy), breeding success, diet, condition and dispersal and recruitment rates. The population on Terschelling numbered 20 pairs, ofwhich 13 had a nest (Tab. 1). In two territories, polygamy was observed. On Ameland seven nests were found in seven territories; one male was paired with three and another one with two females. Clutch size was 4.7 and 3.9 eggs respectively (Tab. 1). Hatching success was highest on Ameland (85% vs. 65% of eggs). Eggs that did not hatch were examined more closely (Tab. 2) and in only two cases evidence for foodshortage was found. The number of young per successful pair was 2.4 and 2.3 young respectively (Tab. 1).
      Diet was investigated by collecting pellets and prey remains around the nest. Voles and birds were the main prey categories based on numbers. Rabbit was the main prey species in terms of biomass (Fig. 1). On Ameland, mainly voles were taken during incubation, while during chick-rearing small birds were the most numerous prey items (Tab. 3). The share of Rabbit remained the same during both periods.
      Compared to historical data, reproductive output and condition of young were not unusual in 2004. Data collected after 2004 however, suggest that poor condition in young does affect breeding success in several years and is caused by a lowprey availability (reduced range of prey species, decline of Rabbits). In this context, it is striking that the population on the large island of Texel is doing rather well. Here, Root Vole Clethrionomys glareolus is an important prey, that is lacking on all the other Dutch Wadden Sea islands (Tab. 4). Besides, dispersal rates and recruitment seem to differ between the islands. Recent colour-ring studies suggest that young hatched on Texel return to the island as a breeding bird, whereas young from other islands often disperse to unknown breeding sites.Moreover, there seems to be immigration fromother breeding sites (one confirmed record from nearby German Wadden Sea islands, where population is thriving). A colour-ring project, started in 2005, will givemore insight in movements between the different breeding sites.

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