Broedende roofvogels op het Friese vasteland: verspreiding, talrijkheid, trend en voedselkeus

The province of Friesland, not including the Wadden Sea Islands, covers 3121 km² of land in the northern Netherlands, mostly flat farmland on clay, peat and sand. The central lake district on peat has a scattering of lakes borderd by reedbelts, with low-lying grasslands in between. The northern and western clay region is largely treeless, and consists of open grasslands and arable land. The eastern region on sand is relatively wooded (the larger forestries up to 1900 ha in size) and mixed farming. Human density in 2005 reached an average of 201 inhabitants/100 ha of land. Using well-studied raptor monitoring plots of 550-20,900 ha (average 4900 ha, N=13), and a wide-ranging network of raptorphiles covering the larger part of Friesland during nest surveys, we are able to quantitatively describe distribution, abundance and trends of breeding raptors during the past two decades. Much information is gleaned from the 1000s of nest cards that have been accumulated over the years. Prey lists have been compiled on the basis of plucks, remains and whole prey found on and near nests throughout the nesting cycle (but mostly in the latter part, hence a bias towards larger prey species). Nine raptor species regularly breed in Friesland, three of which in small numbers: Hen Harrier Circus cyaneus with 0-2 pairs (about to disappear as a breeding bird), Montagu’s Harrier Circus pygargus with up to 5 pairs (both in Lauwersmeer, for Monties the only natural breeding site in The Netherlands), and European Honey-buzzards Pernis apivorus exclusively in woodlands in the eastern half of Friesland with up to 15 pairs. The Marsh Harrier Circus aeruginosus increased in numbers since the early 1970s to some 150-160 pairs in the 1990s, partly a recovery from ravages caused by organochlorines in the 1950s and 1960s. Highest densities of up to 10 pairs/5x5 km occur in the central lake district. Trends are variable, with a steep decline in the Lauwersmeer. This polder was reclaimed in 1969, since when numbers boomed until 1983. Marsh Harriers started to decline here when Red Foxes Vulpes vulpes colonised the area, a decline enhanced by cattle grazing and adverse habitat changes. Elsewhere in Friesland, the increase stabilised in the 1990s, and some regions now show declines in concurrence with habitat deterioration. Food choice in the breeding season is highly varied, mostly consisting of moles, voles, rats and birds (mainly ducklings, young coots, and meadowbirds, fledgling starlings). The proportion of voles is underrecorded, as most nest visits were made during the latter part of the chick cycle. Goshawk Accipiter gentilis, Sparrowhawk A. nisus and Buzzard Buteo buteo increased since the 1970s, spreading from the eastern sand region into the low-lying peat and clay districts, reaching respectively 100, 150-200 and 500-800 pairs in the 1990s. Presently, Goshawks are declining as a result of declining food supply (sandy region) and human persecution (central district). Sparrowhawks reached stable figures in the 1990s, Buzzards in the 2000s. The latter is now widespread and the commonest raptor breeding in Friesland. The diet of Goshawks consisted mainly of birds (73 species, almost 100% in numbers and biomass), with a predominance of ducks, pigeons, thrushes, corvids and starlings. Meadowbirds were taken in low numbers (7.5% in numbers, 1.8% in biomass), of which 47% as full-grown birds. The diet of Buzzards consisted of 18 mammal species, 56 bird species and a variety of reptiles, amphibians and fishes. Mammals predominated the diet (63% in numbers and biomass), especialy moles, voles and hares. Meadowbirds were of minor importance, i.e. 5.2% in numbers and 1.9% in biomass. Kestrels Falco tinnunculus have been largely stable throughout the study period, with some decline in sandy regions (mainly predation by Goshawk and Buzzard, possibly enhanced by food shortage) and higher densities where nestboxes are provided. The 600-700 pairs show fluctuations in line with peaks and troughs in Microtus arvalis populatons. Hobbies F. subbuteo declined on sandy soils (predation by Goshawk and Buzzard), but increased in open farmland where breeding on crow’s nests in electricity pylons is common practice. Numbers do not exceed 80-100 pairs, and the species is thinly distributed, with some indication of decline in recent years. Frisian raptor densities are consistent with findings elsewhere in The Netherlands, accounting for habitat. Friesland shows below-average densities for forest raptors (in line with scarcity of woodland) and above-average densities for open farmland and marshland raptors (consistent with predominance of farm- and marshland).