Bosroofvogels in de IJsselmeerpolders in 1989-95

Raptors breeding in some 13,000 ha of woodland in Noordoostpolder (reclaimed in 1943, planting of woods in 1947-55), Oostelijk Flevoland (1957, planting in 1958-68) and Zuidelijk Flevoland (1968, planting in the 1970s) were studied in 1989-95. Not all forests were equally well-surveyed in all years, but nests were searched for. clutch and brood size determined and nestlings sexed, ringed, weighed and measured whenever possible. The present data are not suitable to calculate densities. Onset of laying was calculated on the basis of the age of nestlings (measured by wing length taken during ringing). Of Honey Buzzard (n=2), Sparrowhawk (n=5) and Kestrel (n=5) few nests were found, mostly because of a combination of unsuitable habitats and insufficient effort to locate nests. Mean onset of laying of Goshawks in 1995 was 7 April, clutch size was 2x 2, 3x 3 and 1x 5 eggs and number of fledglings 6x 1, 10x 2, 17x 3 and 3x 4 young (Table 1). Males predominated among large nestlings (54.5%, based on 402 nestlings and 152 nests) over 1989-95 (Table 2). Pigeons and thrushes constitute the main prey during the breeding season (Table 4). Common Buzzards had an “average” breeding season in comparison with 1989-94, with 5 April as mean onset of laying in 1995, a mean clutch size of 2.8 eggs (5x 2, 10x 3 and 2x 4) and a mean number of fledglings of 2.3 (12x 1, 59x 2, 37x 3 and 7x 4)(Table 3). Secondary sex ratio was established in 20 nests, in which all young were older than 35 days but none had fledged: 26 males and 20 females. Sexing was based on body mass, taking crop size into account. Moles, voles and rabbits were important prey items, but birds were also taken regularly (Table 4).