Zomervoedsel van de Sperwer Accipiter nisus in het zuidwesten van Noord-Brabant, heden en verleden

In the vicinity of the town of Roosendaal in the southwestern part of the province of Noord-Brabant Sparrowhawks were studied in two plots in 1993-98 and in 2017-23, i.e. Rucphense Bossen (1200 ha of mostly coniferous forest, planted in the early 20th century; in 2017-23 only the 800 ha of the northern section of this woodland was covered) and farmland surrounding the woodland plot. Farmland comprised mixed arable land and grassland and was sparsely dotted with woodlots (totaling 150 ha on 25 km² SW of Roosendaal, and 300 ha on 200 km² NW of Roosendaal, the latter woodlots mainly planted in the 1970s and 1980s). Nests were systematically searched for, and checked, from pre-laying through post-fledging. Peak Sparrowhawk numbers in Rucphense Bossen were reached in 1991-92, when 19 and 17 nests were located. Since then, numbers have declined to only 3.4 nests/year in the 2020s (but covering only 800 ha of the 1200 ha surveyed in the 1990s), probably a combination of loss of breeding habitat (especially of spruce, via clear-felling, thinning and mortality caused by Ips typographus) and increased predation by Tawny Owl Strix aluco and Pine Marten Martes martes. Nesting success has strongly declined, i.e. from 81% of 59 nests in 1993-98 fledging at least one chick, to 52% of 24 nests in 2017-23. In farmland, the decline was less evident, and nesting success remained high, i.e. 76% in 1993-98 (67 of 88 nests successful) and 80% in 2017-23 (24 of 30 nests). In the farmland region, nest predation was less often recorded than in the large woodland plot. Prey remains were collected during visits to the nesting site, consisting mainly of pluckings and augmented with skeletal remains (of which especially the humerus was found to be useful in identification) and remains in pellets (of minor importance, and always checked against pluckings). By counting primaries and rectrices, and taking notice of feather development (blood shafts), prey items were individually separated in order to prevent double-counting. The biggest change in Sparrowhawk diets in woodland between 1993-98 and 2017- 23 has been the steep decline in importance (in terms of biomass) of Woodpigeon (respectively 25.5% and 4.5% of total prey biomass), and the loss of Rabbits (resp. 4.6% and 0.0%). Concurrently, particularly Great and Blue Tit, and to a lesser extent also Great Spotted Woodpecker, Robin and Blackbird, increased in importance in diets, perhaps facilitated by the provisioning of nest boxes and woodland becoming more diverse. Sparrowhawks with nests within 250 m of the forest edge showed a higher proportion of House Sparrows and Starlings in their diets than those nesting >250 m from the forest edge or human habitation. In 2017-23 nests within 250 m of the forest edge were also more successful (10 of 14 nests fledging at least one young) than nests farther away (3 of 10 nests successful). Failures in the latter category were mostly due to predation in the nestling stage, when it was also noted that parent birds were often absent during nest checks (indicating that both male and female were away hunting). Diets of Sparrowhawks nesting in farmland were dominated by Starling, House Sparrow and Great Tit. Although House Sparrow has been in decline for some time (though with a small upsurge in recent years), it remains an important prey species, despite strong annual fluctuations in their contribution to individual prey lists. On the other hand, Tree Sparrow has become very scarce in recent prey lists, in line with its steep decline as a breeding bird. The decline of Sparrowhawks in woodland in southwestern Noord-Brabant is probably linked with loss of breeding habitat and increased intraguild predation, rather than with a decline of food resources. Although some important prey species declined in past decades, other prey species like Great Tit and Blue Tit increased and probably compensated for the loss in biomass of – especially – Woodpigeons (and Rabbits). The decline of Sparrowhawks breeding in small and widely scattered woodlots in farmland was smaller, probably due to the fact that predators as Tawny Owl and Pine Marten were scarce in these habitats, or absent.