Trends en broedresultaten van de Grauwe Kiekendief Circus pygargus in Nederland in 2008-13

After an all-time low in the late 1980s, breeding numbers of Montagu’s Harriers started to increase in the early 1990s, a result of EU-driven agricultural set-aside in combination with nest protection. Numbers steadily increased to a peak of 63 pairs in 2011, but with smaller numbers in 2012-13 (Fig. 2). The majority of breeding birds (80% in 2008-13) is restricted to the province of Groningen. Another 10% of the pairs is nesting in extensive arable fields in the province of Flevoland in the central Netherlands, with smaller numbers nesting in northern Friesland (in total 12 pairs in 2008-13), Drenthe (3), Overijssel (2) and Noord-Brabant (1) (Fig. 1). Habitat choice has been studied in detail in Groningen, where Montagu’s Harriers have been equipped with a datalogger. These birds mainly hunted over recently mown grasslands, rather than in set-aside fields. The paradox of an increase in numbers in the footsteps of set-aside measures versus an ‘avoidance’ of set-aside as hunting (and breeding) haunt is only an apparent paradox: set-aside is a source of voles, which colonise the neighbouring fields and grasslands where subsequently voles become available to harriers after mowing. The effect of set-aside is therefore secondary, but no less important. As nesting habitat, winter wheat was used by 75% of the pairs in 2008-13, the remaining pairs nested in other crops like winter barley, alfalfa and coleseed rape (18%), and semi-natural habitats like saltmarshes, reedbeds and forestry plantations (7%). The present nesting habitat, i.e. mainly farmland, signifies a shift compared to the 1980s, when most pairs bred in semi-natural vegetations and forestry plantations. This also explains why protection measures remain important to ensure safety to nests. Mean start of laying varied between 17 May and 14 June, depending on year and region. For 168 pairs in 2008-13, start of laying averaged 24 May (SD 7.7). Mean brood size varied between 0.8 and 2.3 per successful pair, with lowest breeding success in years with low vole numbers. Overall, the secondary sex ratio as obtained when ringing nestlings was in favour of males (56.3%, N=360 chicks on 295 nests), but with clear annual variations in the proportion of males (apparently in line with vole numbers: fewer males in years with high vole numbers). Over the years, thousands of prey remains were collected at and near nests (4473 prey items in 2008-13). Common Voles Microtus arvalis were by far the most important prey species, accounting for some 60% in numbers and 85% in biomass (Fig. 3). In years with low vole numbers, birds (notably Yellow Wagtail Motacilla flava, Skylark Alauda arvensis and Meadow Pipit Anthus pratensis) increase in importance, with up to onethird in numbers in extremely poor vole years like 2013 (Appendix 1 and 2). Insects were often taken, but were of lesser importance in terms of biomass.