Considerations on Demography and Conservation of Montagu's Harrier Circus pygargus in east Groningen, Netherlands

We studied correlations between reproductive parameters and environmental factors in Montagu's Harrier, breeding in agricultural habitat in east Groningen, Netherlands. Knowledge on such factors is of importance for conservation of the marginal population there. Common Voles (Microtus arvalis) made up between one third (considering estimated biomass) and half (considering prey number) of the diet. Diet was diverse, indicating generalist behaviour in 2003. During the study period (1992-2003), 1992 was the only good vole year. Voles were most numerous on fallow land and in high vegetation. Montagu's Harrier in east Groningen exhibited no numerical response to vole abundance during the study period. There was a trend of earlier laying in relatively good vole years, but other breeding parameters did not correlate with vole abundance. This and the generalist behaviour indicate that Montagu's Harrier was not single prey dependent in poor vole years. Breeding variables did not correlate with weather parameters. Vole abundance on the other hand correlated with maximal temperature and sunshine duration (average April-August). In protected nests, clutch sizes were larger and egg-fledgling survival better, but not significantly. We conclude that nest protection is still crucial but not effective on the long run. Habitat management aiming at better food supply, e.g. more extensive agriculture, setaside edges with high vegetation and mowing management, is more likely to increase breeding pair numbers. Identifying and protecting most productive areas in the population supposedly covering the Netherlands, Germany and Denmark, could favour a sustainable sub-population in east Groningen.