KOKS BJ, VAN SCHARENBURG CWM & VISSER EG (2001) The Montagu's Harrier Circus pygargus in the Netherlands: balancing between hope and dispair. LIMOSA 74 (4): 121-136.

The Monlagu's Harrier was threatened to become extinct as a breeding bird in the Netherlands towards the end of the nineteen eighties. The province Flevoland was by far the most important region for the species between 1975 and 1989 (Fig. 2a). However, the setting aside of thousands of hectares of arable land, in particular in the Oldambt region of Eastern Groningen, created a situation that was attactive to the Montagu's Harrier and other mouse-eating birds of prey because of high densities of Common Voles. Once the amount of set aside arable land began to decrease, the number of breeding pairs Initially declined. However. the population more or less stabilised from the middle of the nineties onwards (Fig. 1). The breeding population o' eastern Groningen reached a maximum of 31 pairs in 2000.
      From about 1990 onwards most nests have been found in lucerne and winter wheat crops (Fig. 3b). Between 1975 and 1989 many more birds bred in nature reserves and tree plantations (Fig 3a). Nowadays, the Lauwersmeer is the only remaining Dutch breeding area of any importance in which the species breeds In a semi-natural environment.
      In 1990-2000 the number of eggs laid by the population in Groningen averaged at 3.89 ± 0.11 and the average number of fledged young was 1.51 ± 0.17 young per successful pair (1.24 when based on the Mayfield method), slightly less than elsewhere in Europe (Tab. 1). Important causes of loss are human disturbance (52%) and predation (40%). In eastern Groningen the average number of fledglings is doubled when nests are protected from destruction due to agricultural activities Comparison with survival estimates from southern Europe suggests that breeding productively (Tab. 2) fully compensates tor mortality only In protected nests in eastern Groningen. I1 seems that the Dutch population could not persist it the nests ""ere not protected in arable land (Fig. 6). Since 1992, the trend in numbers counted in Groningen was more positive than predicted from a model based on observed reproduction and assumed normality, suggesting that immigration took place. We believe that the populations in Germany and Denmark act as a source of birds for the small Dutch population. This suggests that protection bath here and elsewhere, as well as adequate supervision on a large scale, is necessary 10 maintain a stable population.
      Nowadays, the diet of Montagu's Harrier in eastern Groningen differs from that during the sixties and seventies of last century. Although this opportunist still captures a broad spectrum of prey , the share of Common Voles has increased. In Groningen the proportion of voles in the die! fluctuate between 15% and 74% and there was a positive correlation between the proportion voles and both clutch size and the number of fledglings. Without nest protection. the positive effect of high vole abundance on productivity was not expressed (Fig. 5).

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