Lange-termijn veranderingen in dichtheid en reproductie van Haviken Accipiter gentilis in een sterk door mensen beïnvloed landschap

Population dynamics of Goshawks were studied in a forestry in the northern Netherlands (Hooghalen, 2010 ha, 60% coniferous, rest heath- and farmland) in 1983-2011. After an initial increase from 9 pairs to 10-12 in the late 1980s and 1990s, numbers declined to 4-8 pairs after 1999. The steeper ups and downs in the last decade are probably due to an increase in the number of non-breeding pairs in some years, concurrent with a lower profile (less vocal). Over the entire period, 245 nests in 249 territories were located; egg-laying commenced in 212 nests (85%), hatching occurred in 186 nests, and 175 nests were successful in raising one or more fledglings. Up to 50% or more of the breeding females were in juvenile plumage during the first few years of the study, reflecting the growing population and human persecution (high mortality). From the early 1990s onwards, almost all breeding females have been adult (in 16 of 18 years; see Appendix). Onset of laying ranged from 24 March to 29 April, but mean start of laying did not vary between the early period of study (1983-92), the central period (1992-2002) and the late period (2003-2011), with a mean start of laying of 5, 5 and 6 April respectively. However, the laying period in the 2000s was more protracted than in the two earlier periods (10-12 days shorter). Onset of laying showed a clear negative correlation with mean March temperature. Clutch size was 4x 1, 20x 2, 104x 3, 68x 4 and 1x 5 (mean 3.2/pair), and showed a rather abrupt reduction after 1990. Clutch size declined with laying date, but not with decreasing temperature. On average, 3.0 chicks hatched per nest (8x 1, 37x 2, 74x 3, 47x 4, lx 5). Up to and including 2002, hatching success did not differ between large and small clutches, but thereafter declined with declining clutch size. Growth speed of wing length declined over time, both for male and female chicks (using the last-taken measurement when wing length was measured more than once), and compared to growth of the A-chick. For B-chicks the speed of wing length growth declined by 4% between 1983 and 2011 (extending fledging date by 1.5 days), and by 8% for C-chicks (3 days ditto). Relative body mass of both sexes also showed a decline over time, and more so in females than in males. The number of fledglings averaged 2.6 young in successful nests, and 1.8 young in all nests. Chick mortality increased over time, resulting in a decline in productivity over time. The secondary sex ratio of nestlings was in favour of males (58% of 411 chicks). The proportion of males increased with clutch size: 47% in C2 (N=17), 56% in C3 (N=204) and 63% in C4 (N=181). Sex ratio did not correlate with onset of laying, but a calender-effect was clearly visible with 50% males in 30 March-8 April (N=243), 60% in 9-18 April (N=72) and 61% in 19-28 April (N=18). The proportion of males increased with brood size, an indication of female-biased mortality in larger broods. The decline in density, breeding performance and chick condition since the early 1990s was likely caused by a long-term decline in food supply, notably of species favoured by Goshawks as food (like ducks, galliformes, pigeons, thrushes, corvids, starling and so on). This decline was quantified for winter food, and probably also held for food supply in summer. At present, density and reproductive output are similar to the average values for much of western Europe.