Beperkt het terreingebruik van de Bosuil Strix aluco de nestplaatskeuze van Sperwers Accipiter nisus?

In the 1970s Sparrowhawks were rare breeding birds in the forest of Rucphen (SW-Netherlands, 830 ha of which 650 coniferous, rest heath and farmland) and smaller woodlots nearby. In 1989, at least 7 nests were present, further increasing to 13 and 12 nests in 1991 and 1992. In 1993-2002 the number of nests averaged 8 per year (range 6-10, SD=1.8), i.e. back to the level of the early 1990s. When this area was surveyed again in 2012-15, Sparrowhawks seemed to have disappeared, with no nests, moulted feathers, prey remains or other tell-tale signs in tree stands suitable for nesting. However, in 2016-19 a total of 13 nests were located, mostly in atypical stands (like young Pinus sylvestris); in the past, most nests had been built in Douglas fir and Norway spruce (66% of 77 nests, whereas fir/spruce amounted to only 5% of the total acreage). In 1993-2002, out of 64 nests, 72% hatched at least one egg (and 70% fledged at least one chick). In 2016-19, 11 out of 13 nests hatched at least one egg, but only five nests were eventually successful in fledging one or more chicks. Nests in spruce/fir were more successful (3 out of 5) than those in Scots pine (0 out of 4). Several cases of predation were recorded in 2016-19, by Goshawk Accipiter gentilis and Tawny Owl. Tawny Owls only recently colonized Rucphense Bossen, with no territories in 1989 and with just four sites with begging owlets in 1999. In the northern part of Rucphense Bossen 15-20 territories were located in 2016, showing a vast increase in numbers. Night time roosts, characterized by faeces, pellets and moulted feathers, were usually situated in not too dense woodland of spruce/fir (with high density of Wood Mouse Apodemus sylvaticus). Such roosts were never occupied by owls during day time. The high Tawny Owl density in combination with their preference for night time roosting in stands of fir/spruce potentially conflicts with nest site choice of Sparrowhawks. Several killings of Sparrowhawks (chicks, adult) suggested Tawny Owls as the predator. However, when Sparrowhawks avoided night time roosts of Tawny Owls in spruce/fir stands as a nesting place, successful breeding was still possible.