Invloed van grootschalige boskap op broedende roofvogels

Between 1990 and 2020 the numerical and reproductive performance of a raptor population in West Drenthe were systematically monitored (36,300 h on 7274 field days). The study area covers 4660 ha (some 2500 ha of woodland, rest heaths and farmland) and is managed mainly by State Forestry and a private nature conservation society (Natuurmonumenten). In 1990, some 52% of the woodland consisted of non-native tree species, mostly Picea abies (18.9%), Larix leptolepis (16.1%) and Pseudotsuga menziesii (7.4%). Among native tree species, Pinus sylvestris (39.2%) and Quercus robur (6.7%) were most important. This region was converted into a National Park in the year 2000, since when stands of non-native trees have been clearfelled on a grand scale, in conjunction with the introduction of large herbivores (cows and sheep) and promotion and facilitation of mass recreation. Since 2000, Forestry of Smilde and Berkenheuvel respectively lost 36% and 20% of their forest to clearfelling, almost exclusively non-native trees, i.e. -99% for Pinus contorta, -76% for Picea sitchensis, -75% for Quercus rubra, -69% for Larix leptolepis, -63% for Picea abies, -61% for Pinus nigra nigra and 54% for P. nigra laricio and Pseudotsuga menziesii. Remaining forest became fragmented over extensive surfaces. Recreational activities have increased up to 30-fold in just two decades, nowadays covering the entire year (in the 1990s, mostly restricted to the summer season and holidays). Mass recreation increasingly conflicts with protection of safe breeding sites for raptors. Annually since 1990 the forests have been intensively surveyed in order to locate raptor nests, collect prey remains and moulted feathers, and identify territories. All trees with occupied nests were repeatedly climbed within the breeding season to ascertain clutch and egg size, to age, sex and ring nestlings, take morphometrics of nestlings (including crop size), collect and identify prey remains and establish nest success. The efficacy of finding nests was tested in July-August when prolonged treetopping sessions throughout the forest were used to detect Honey Buzzard nests via food transportation flights (meanwhile keeping an eye on fledglings and behaviour of other raptor species), and also by checking stands in winter to validate observations during the previous summer (especially when a nest was not yet located) and to check condition of known nests. In 1996, before the National Park was created, a total of 207 large raptor nests were known (built by medium-sized raptors), of which 57 were occupied (5 Pernis apivorus, 36 Buteo buteo, 16 Accipiter gentilis). Twenty years later, in 2016 (and 16 years after the birth of the National Park), the number of nests had declined to 61 (i.e. 29% of the 1996-number, of which 27 were occupied: 3 P. apivorus, 19 B. buteo and 5 A. gentilis). The massive decline was concomitant to the simultaneous clear-felling of non-native trees, because all raptor species had shown a significant preference for nesting in non native trees, i.e. the tree species specifically targeted for logging by nature conservationists: Honey Buzzard for Picea abies, Goshawk for Larix leptolepis and Pseudotsuga menziesii, Sparrowhawk for P. abies, P. sitchensis and Pseudotsuga menziesii and Buzzard for L. leptolepis. All species were significantly less often recorded nesting in most of the native tree species than expected from a random choice. Longevity of nests depended on tree species, with nests in non-native trees showing longer lifespans: on average 9.7 years in L. leptolepis (n=99, range 2-27 years), 6.8 years in Pseudotsuga menziesii (n=15, 1-28) and 5.5 years in Picea abies (n=46, 1-14) compared to 5.2 years in t he native Pinus sylvestris (n=86, 1-15). The lifespan of raptors nests was significantly reduced with large-scale clear-felling, for example by three years in Larix leptolepis (from 11 to 8 years). Logging and clear-felling also impacted nest use in afflicted stands. Without logging Goshawks successively reused the same nest up to 11 years (normally between 1 and 6 times), but logging resulted in displacements of the breeding pair in 15 out of 23 cases (8 of which associated with loss of breeding pair). Between 1990 and 2020 all breeding raptor species significantly and consistently declined, even by 100% for Kestrel and Hobby which had already disappeared by the mid-1990s and had been scarce breeding birds anyway. The other species declined by 50-55% (Honey Buzzard and Buzzard, with maxima of resp. 8 and 43 pairs), 60% (Goshawk, maximum of 16 pairs) and 85% (Sparrowhawk, maximum of 17 pairs). Declines only became prominent after the region was declared a National Park, and accelerated when clear-felling of non-native trees gained momentum. The decline in numbers was paralelled by a decline in nesting success, which was particularly steep in Sparrowhawk (46% successful in 1990-99, only 13% in 2010-19) and Goshawk (73% successful in 1990-99, reduced to 52% in 2010-19) but less so in Honey Buzzard (55% and 46% respectively). Nesting success of Buzzards remained more or less stable (varying between 56% and 63% per decade in 1990-2019, but notice that the number of breeding pairs had halved in the intervening period; territories with scant food supply have been abandoned). For all species, nest predation is (or has become) an important cause of failure. In the 1990s nest predation was restricted to the smaller raptor species, i.e. Hobby and Sparrowhawk, and to Honey Buzzard, and spasmodically affected up to half of the breeding populations. Since the year 2000, nest predation started to occur in Buzzards as well, followed by nest predation among Goshawks from 2007 onwards. Predation nowadays affects the majority of nests of Sparrowhawk and Honey Buzzard (in some years all nests, often including one or both parents), and between 30 and 50% of all Buzzard and Goshawk pairs. Goshawks are most frequently associated with nest predation of raptor nests (but also Pine Martens Martes martes, Buzzards and Tawny Owls Strix aluco; the recent colonization of the National Park by Eagle Owl Bubo bubo in 2019 added another potential nest predator), despite the fact that breeding numbers of Goshawks have more than halved in the same time span (hence many fewer Goshawks present). The decline in nesting success in Goshawks was mirrored by a delay in laying date, a decline in clutch size (but not in egg volume) and a decline in the number of chicks raised to fledging (for successful pairs as well as for all pairs). No such consistent reproductive trends were found in Sparrowhawk (no long-term change in laying date or clutch size) and Buzzard (ups and downs in laying date and clutch and brood size were associated with vole numbers). The increase in predation rate, especially by Goshawks, is probably linked to a steep decline in food supply. Sixteen prey species in the category of 75-500 g consistently declined in biomass between the 1960s and 2010s, in the latter decade only some 20% was left of the initial bonanza. Similarly, racing pigeons Columba livia (a major prey species of Goshawk in summer) has substantially declined, as did Rabbit Oryctolagus cuniculus. The avian biomass in winter has declined even more steeply. The decline in food abundance (and availability) was further exacerbated by largescale felling (habitat destruction). Raptor losses due to logging were also aggravated by the inadequate implementation of the code of conduct adopted by forestry personnel and nature managers, which prescribed a buffer zone of 50 m from raptor nests in stands to be harvested. In only 72 of 141 nests of Honey Buzzard, Goshawk, Sparrowhawk and Buzzard the nest had been located prior to felling. In 25 of these 72 nests the buffer zone was neglected (trees logged up to nesting tree), the remaining nests had been marked with a buffer zone of at most 35 m (instead of 50 m) and often much less, and further inflated during harvesting. Of marked nests, 21-100% were eventually felled during harvesting, the proportion depending on raptor species (highest for Honey Buzzard and Sparrowhawk, lowest for Goshawk). Overall, combining marked and unmarked nests, after logging only 17% of Sparrowhawk nests remained, 18% of Honey Buzzard’s, 38% of Buzzard’s and 66% of Goshawk’s. The code of conduct is used as a method to circumvent the law (all raptor species are protected by law) and to allow forestry activities during the breeding season. Enforcement of the code of conduct is in fact non-existent, which explains the failure to protect nest and nest sites of raptors (not to mention the fact that a buffer zone of 50 m is no protection anyway when the surrounding forest is clear-cut, as evident from the high predation rates). The steep raptor declines recorded in West-Drenthe can be attributed to large-scale logging, aggravated by equally steep declines in food supply (following logging, and huge loss of biodiversity in the surrounding farmland). The conversion of nature reserves into recreation areas added insult to injury.