Trends en broedresultaten van roofvogels in Nederland in 2000

A total of 4352 raptor nests were registered in 2000, covering 11 species (Fig. 1, Appendix 1) and most regions and habitats within The Netherlands (Fig. 1). Basic data taken from these cards are presented here, i.e. clutch size (full clutches only), number of fledglings (usually recorded during ringing, unless a later nest visit was paid; nestling counts from the ground were not included), sex ratio (based on nests where all surviving young were sex-identified and weighed/measured) and onset of laying (mostly back-calculated from wing length=age and controlling for clutch size). Food items collected during nest visits are presented in Appendix 9 (Goshawk) and 10 (Common Buzzard). During 2000, at least 8426 raptor nestlings were ringed (Table 1). The winter of 1999/2000 was very mild (Ijnsen frost index 3.6), the summer was rather warm (Unsen summer index 57.9). A gale on 28/29 May caused havoc among nesting raptors, with 124 nests (out of 4352) damaged to such an extent that all nestlings/eggs were killed, especially among Common Buzzards (4.6% of all nests) in farmland and Sparrowhawks (4.1%); partial losses were numerous. Honey Buzzard Pernis apivorus: a very successful breeding season, thanks to large numbers of social wasps through early August (German wasp Vespula germanica) and late August/early September (common wasp V. vulgaris). Onset of laying was on average on 22 May, the first clutch being initiated on 15 May (n=36, Table 3, Appendix 2)! Clutch size was 2.0 (n=18, Appendix 3), number of nestlings/- successful pair on average 1.8 (n=35, Appendix 4), The number of non-laying pairs was very small, i.e. 5 out of 51. In an area in Drenthe (4466 ha, of which 64% woodland), where Honey Buzzards are studied using standardized methods, nonlaying varied between 17% (in 2000) and 80% (in 1997; Table 2). Several nestlings in 2000 weighed >1200 g, proof of superior food and feeding conditions during summer. Black Kite Milvus migrans: a breeding attempt along the river IJssel failed during the egg stage (Schoppers 2000). This was the third breeding attempt in The Netherlands ever. Marsh Harrier Circus aeruginosus: mean onset of laying was 23 april (n=140, cf. Appendix 2), mean clutch size 4.6 (n=89, Appendix 3) and mean brood size 3.2, n=154, Appendix 4). Variation between regions was small (Table 4). In 119 nests all young were sexed: 185 males and 184 females. Hen Harrier Circus cyaneus: the Dutch population is dwindling, with viable subpopulations at the Wadden Sea Islands of Texel and Terschelling only. The species almost disappeared from mainland breeding haunts. It is steeply declining on the Wadden Sea Islands of Ameland and Schiermonnikoog as well. On Ameland, for example, the number of breeding pairs declined from 27 in 1990 to 4 in 2000 (raising only 7 young by 3 pairs; cf. Figure 2). Mean onset of laying was 3 May (Appendix 2), mean clutch size 4.0 (Appendix 3) and mean brood size 2.4/successful nest and 1.7/pair (Appendix 3). It is thought that adverse habitat changes are responsible for the decline and poor breeding success, probably also poor first-year survival (based on analysis of ring recoveries; M. Lof). Montagu’s Harrier Circus pygargus: the data covering 2000 were published by Koks & Visser (2000). Of 43 known nesting sites, 34 were in cropland, the remaining in nature reserves, rough land and forestry plantation. Nest protection was therefore necessary to guarantee successful fledging. Mean onset of laying was 22 May (4 May-15 June, n=23, Appendix 2), mean clutch size 3.6 (Appendix 3), mean brood size 2.6 (Appendix 4). Sex ratio was 26 males and 29 females. Goshawk Accipiter gentilis: mean start of laying was 2 April, with the first egg on 16 March (n=360, Appendix 2). On average, Goshawks in the southern Netherlands started a few days earlier than those in the northern and eastern Netherlands (Table 5). Adult pairs started egg-laying on 2 April on average (n=41, SD=7.1), much earlier than pairs in which the female was in first-year plumage (12 April, n=14, SD=10.7; males either adult or age not known). Of aged breeding birds 2 out of 56 males were in first-year plumage; in females this was 18 out of 171 (10.5%). This proportion is slightly increasing during the last few years, probably indicative of a higher turnover in some parts of The Netherlands (persecution, increasing populations in the western Netherlands). Goshawks in parts of Drenthe and on the Veluwe may experience food shortage (resulting in higher turnover?); depredation of Goshawk nestlings by Goshawks, until recently unknown or rarely recorded, seems to be increasing in frequency (as is depredation of Common Buzzard nestlings by Goshawks). Clutch size averaged 3.4 (n=250, Appendix 3), brood size 2.7 (n=436, Appendix 4). Sex ratio in 325 nests was in favour of males (57.3%, Table 6), i.e. consistent with earlier results (overall in 1996-2000: 54.9% males among 3971 nestlings on 1476 nests; Table 6). Sparrowhawk Accipiter nisus: mean start of laying was 30 April (n=269, Appendix 2), with a later start in the western Netherlands (where population is still increasing and a higher proportion of first-year birds is involved) and in Flevoland (high proportion nesting in deciduous woodland, where a delay is probably related to leafing of trees) (Table 7). Mean clutch size was 4.8 (n=215, Appendix 3), mean brood size 3.9 (n=314, Table 4). Seven clutches contained 7 eggs, a higher number than in preceding years: 0 in 1996 (n=131) and 1997 (n=272), 3 in 1998 (n=298) and 4 in 1999 (n=260). Adult pairs had an earlier start (27 April, n=14, SD=5.8) than pairs consisting of adult male and first-year female (29 April, n=2, SD=2.0). The only recorded pair in first-year plumage started on 2 May. Nestlings were sexed on 256 nests (50.3% male, 998 nestlings); in the period 1996-2000 the overall sex ratio on 1310 nests was also 50.3% (Table 8). An important cause of failure was predation by Goshawks. Common Buzzard Buteo buteo: the commonest raptor in The Netherlands, even occupying open farmland and built-up areas. This distribution is also reflected in unusual nesting sites, such as a ground nest in farmland (Friesland), two nests in electricity towers and nests in solitary trees in open farmland. Mean onset of laying averaged 5 April (n=855, range 12 March-14 May; Appendix 2). The relatively late start of laying in Friesland (8 April, as compared to 3-4 April in adjacent Drenthe and Groningen; Table 9) is proof of extensive raptor persecution, and therefore of a higher proportion of repeat layings (the latter started on average on 14 April). Mean clutch size was 2.46 (n=549, Appendix 3), mean number of fledglings/successful nest 1.94 (n=1063, Appendix 4). Despite the fact that 2000 was not a vole-year, 2 clutches with 5 eggs were recorded (normally only in vole-years). A clutch with 6 eggs was possibly produced by two females; this nest resulted in 5 nestlings (see photographs). In 270 nests all nestlings were sexed: 251 males and 224 females (Table 10). Causes of failure were mainly human-related (in Friesland mostly caused by protectionists of meadow birds!). The storm of 28/29 May also damaged many nests (77 completely lost, at least 11 with partial losses). Kestrel Falco tinnunculus: clutches were initiated between 25 March and 1 June, on average on 25 April (n=701, Appendix 2). Regional variation was small (Table 11). Mean clutch size was 5.21 (n=621, Appendix 3), mean brood size was 4.30 (n=857, Appendix 4). Pairs nesting on crow’s nests failed twice as often (11 out of 47) than pairs nesting in nestboxes (101 out of 962), mainly because of predation. Hobby Falco subbuteo: Hobbies have become scarce in most wood- and heathlands in the eastern and central Netherlands (formerly the main breeding haunts). In 2000, most Hobbies were located in farmland in Friesland, Noord-Brabant and Limburg (Table 12). Of 104 nests located, 13 were in crow’s nests in electricity towers. Main nest suppliers are carrion crows (89% of 100 nests). Onset of laying averaged 12 June (n=63, Appendix 2), mean clutch size was 2.83 (n=29, Appendix 3), mean brood size 2.30 (n=86. Appendix 4). Sex ratio (based on nestlings >23 days old, using body mass and/or call) in 22 nests was 26 males and 26 females. Main causes of failure were predation (7x nestlings, lx parent), adverse weather (storm of 28/29 May, 2x) and desertion (2x). Prey items found on/near nests were identified in three provinces (Table 13). Slechtvalk Falco peregrinus: the Dutch population increased from 6 pairs in 1999 to 7 pairs in 2000. All pairs used specially provided nestboxes on industrial sites. Clutch size was 2x 3 and 5x 4 eggs, brood size 2x 3 and 4x 4 nestlings (one nest failed). All nestlings were ringed (including colour-rings); sex ratio in 6 nests was 11 males and 11 females. Mean onset of laying was 10 March (range 27 February-2 April) (Table 14). The newly settled pair in the western Netherlands had a wellstocked larder near its nest, containing many nocturnal migrants such as Moorhens, Water Rails and grebes, and also many Snipes and Teal (van Geneijgen 2000). This pair probably benefited from the brightly illuminated industrial site, which attracted large numbers of nocturnal migrants.