Roofvogels als trendwatchers van de stand van konijnen

Koning F. & Koning H.-J. 2024. Raptors as trendwatchers of Rabbits Oryctolagus cuniculus. De Takkeling 32: 101-108. Breeding raptors and owls, and their prey, have been monitored in the Amsterdamse Waterleidingduinen (3500 ha) since the late 1950s. Monitoring of Rabbits started in 1985, i.e. long after the crash in numbers in the 1950s (myxomatosis). Since the myxomatosis crash numbers initially recovered till at least the early 1990s after which Rabbit Viral Haemorrhagic Disease (RVHD) kicked in and numbers again plummeted. Habitat destruction via clear-felling and removal of Prunus and undergrowth with heavy machinery further reduced the prey base of raptors and owls in the 21st century. The long-term fortunes of Rabbits were reconstructed via prey lists of Tawny Owl Strix aluco (n=18,379 prey items, 100-600/annum) and diurnal raptors. Tawny Owl colonized the dunes in 1961, then increased to 25-32 pairs from the 1970s to the mid-2010s, and declined thereafter including a disappearance from the central dunes due to habitat fragmentation in the wake of clear-felling. Rabbits used to be an important prey for Tawny Owls up through the 1970s, then steeply declined when Red Foxes colonized the dunes (1978, but probably in 1975) and increased to a density of 30-40 dens in the 1980s and 1990s. Fox diet during this period contained a high proportion of Rabbits (up to 75%). Later on, the dunes were colonized by Buzzard Buteo buteo (in 1989, increasing to 15-20 pairs in 2000-20, up to 80% Rabbit in summer diet) and Goshawk Accipiter gentilis (1993, increase to 10-13 pairs till early 2020s, up to 30% Rabbit in summer diet). Pine Martens are present since 2003, but their diets contained few Rabbits (8 out of 327 prey items, and none after 2017). The total predation pressure, in combination with repeated outbreaks of RHVD and – from the 2010s onwards – systematic habitat destruction (clear-felling, removal of undergrowth, overgrazing), has led to steep declines in the Rabbit population, with a concomitant decrease of Rabbits in diets of Tawny Owl, Buzzard and Goshawk, and eventually in declines of the populations of Tawny Owl and diurnal raptors. After the Rabbit crash, Tawny Owls diversified their diets with birds and – when these declined by 70% following ‘nature restoration projects’ later on – with amphibians. The present decline of Tawny Owls, diurnal raptors and Red Foxes is a direct effect of dwindling food resources (notably Rabbits via predation and RVHD, and rodents and birds via removal of vegetation and overgrazing) in combination with destruction of breeding habitat.