Systematisch roofvogelonderzoek in Drenthe in 1995

From 1984 onwards, raptors in several study areas in the province of Drenthe have been studied quantitatively by means of standardized mapping, nest searching, controlling nests, ringing, taking biometrics, identifying age of territory-holders, sexing nestlings and searching for prey items. Not all sites have been studied in each year over the period of 1984-95. Those studied in 1995 are depicted in Table 1, covering 146 km2 of which 23% is afforested. Honey Buzzards received special attention by a few specialists, who covered almost the entire province. 13 nests were found in 25 territories (Table 2); 19 young fledged from 11 nests (8x 2, 3x 1). Mean onset of laying in 10 nests was 24 May (SD=4.2, range 19-31 May). Nesting trees were Larix leptolepis (6), Pseudotsuga menziesii (4), Picea sitchensis (1), Pinus sylvestris (1) and Quercus robur (1). Nests were located in 91% of 77 territories of Goshawks; at least 83% of these pairs produced clutches, in 65% of the pairs eggs hatched and in 63% of the pairs young fledged. The majority of breeding birds were >2 years old (97% of 29 males, 91% of 65 females), indicating a stable population. Secondary sex ratio was in favour of males (58% of 107 young at ringing age). Onset of laying, clutch size and number of fledlings (inclusing SD, range and numbers) over 1984-95 are specified in Table 3. Nests were located in 96% of 55 territories of Sparrowhawks, resulting in egg-laying in 93%, hatching in 76% and fledging in 62% of the cases. Most breeding birds were adult: 90% of the males (n=20) and 91% of the females (n=45). Onset of laying, clutch size and number of fledglings (including SD, range and numbers) are given in Table 4. Secondary sex ratio among 139 nestlings was 54% in favour of males. Common Buzzards are the most abundant avian predators, with 179 territories found in the study plots; nests were located in 76% of these territories and 61% of the pairs started egg-laying. Hatching and fledging occurred in 51 and 45% of the territories respectively. Very few breeding birds were in first year plumage: 0.8% of males (n=124) and 1.4% of females (n=140). Onset of laying look place between 30 March and 22 April (mean 9 April, Table 5), mean clutch size was 2.2 eggs and mean number of fledglings in successful pairs 1.7, the latter value being the lowest in the period 1984-95 (Table 5). Mean clutch size in Kestrels was 5.2 (SD=0.9, range 3-7, n=43), mean number of fledglings 4.2 (SD=1.3, n=41) and mean onset of laying 1 May (SD=7.2, n=43, range 18 April-17 May), despite a low in vole numbers. Hobbies have almost disappeared from the study plots in Drenthe, only 4 territories being found in 1995 (2 nests: one failed, another raised 3 nestlings with 1 June as onset of laying).