Trends en broedresultaten van roofvogels in Nederland in 2001

Despite the outbreak of foot-and-mouth disease (FMD) in mid-March 2001, leading to extensive areas being out of bounds for field workers through 24 May, the majority of raptorphiles were still able to cover their study sites adequately. The 4362 submitted nest cards of 10 raptor species are proof that the observer-effort was similar to that in the preceding years (Appendix 1; number of nest cards per species per province). The smaller number of nestlings ringed as compared to 2000 (Table 1) was mainly due to smaller brood sizes in most species, and to some extent also to the side effects of FMD. Basic data taken from nest 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 backcalculated 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). The winter of 2000/2001 was mild, with Ijnsen’s frost index at 8.6; the summer of 2001 was categorised as warm (Ijnsen’s summer index: 70.8). Rabbit populations were at a low ebb (probably still suffering from outbreaks of VHS), and numbers of hares were also small. Vole indices showed poor (Groningen, Flevoland) or moderate population sizes, although locally populations of wood mice Apodemus sylvaticus and bank voles Clethrionomys glareolus still abounded in late winter, spring and early summer as a result of good crops of beech and oak in autumn 2000 (both 4 on a scale of 0-5). Social wasps were present in small numbers (2 on a scale of 1- 5), with German wasps Vespula germanica peaking in late July and common wasps V. vulgaris somewhat later. Honey Buzzard Pernis apivorus: despite a relatively warm May (mean temperature 14.1°C, mean value in 1961-90 = 12.3°C), mean onset of laying was 28 May (range 21 May-9 June; Appendix 2), i.e. 6 days later than in 2000 (when May temperature averaged 14.7°C). Twelve out of 35 nests failed, mostly because egg laying did not commence but also following egg desertion and nestling predation (by Goshawk Accipiter gentilis). Mean clutch size was 1.94 (1x 1,15 x 2; Appendix 3), mean brood size 1.54 (1Ox 1,12 x 2; Appendix 4). In two areas of resp. 6880 and 4466 ha, where Honey Buzzards are being studied using species-specific methods, a slight but consistent decline was noticed in the 1990 s (Fig. 1), Marsh Harrier Circus aeruginosus: mean onset of laying was 27 April (Appendix 2), mean clutch size 4.79 (Appendix 3) and mean brood size 3.54 (Appendix 4), with an overall earlier start in various parts of the country (Table 2) than in 2000. The secondary sex ratio was in favour of males (52.7%, based on 300 nestlings in 86 nests, i.e. in line with results in 1997-99 but unlike 2000 (Table 4). Prey remains found on nests are summarised in Table 3, showing the wide diversity in food choice and a complete overlap with that of Common Buzzards Buteo buteo. Whereas these species were entirely allopatric in the 1970s (Buzzards in woodland in the eastern half of the country. Marsh Harriers in wetlands in the western half), the overlap in habitat is nowadays complete and direct food competition is likely. Hen Harrier Circus cyaneus: numbers breeding on the Wadden Sea Island of Ameland reached with 3 pairs (two pairs raising 6 young) an all-time low, as on Vlieland (4 pairs, only one pair fledging 4 nestlings). On the other hand, Texel (66 nestlings ringed) and Terschelling still hold substantial numbers, to aiesser extent Schiermonnikoog as well. Nevertheles, the species is in dire straits because of declining survival rates in adults (Lof 2000) and low reproduction rates. Mean onset of laying was 1 May (n=13, range 18 April-12 May: Appendix 2), mean clutch size 4.88 (n8; =Appendix 3) and mean brood size 3.42 (n=12; Appendix 4), with a secondary sex ratio of 22 males and 19 females (on 12 nests). Food remains collected at a nest on Vlieland are tabulated in Table 5, showing a wide array of birds and some mammals (mainly rabbit Oryctolagus cuniculus). Montagu’s Harrier Circus pygargus: a detailed report on this species can be found elsewhere in this issue of De Takkeling and on www.sovon.nl. Mean onset of laying was 22 May (Appendix 2), mean clutch size 3.47 (Appendix 3) and mean number of fledglings 2.64 (Appendix 4). Secondary sex ratio on nests where all young were sexed was 16 males and 18 females. Most pairs were breeding in the province of Groningen (n=27), smaller numbers in Friesland (Lauwersmeer, n=4), Flevoland (n=3) and Drenthe (n=1). Goshawk Accipiter gentilis: Goshawks are still slightly expanding their breeding range, particularly in the western Netherlands and on the Wadden Sea Islands (with 10 pairs on Texel and 2 pairs on Vlieland in 2001). Mean onset of laying was 31 March (range 11 maart-30 April, n=358; Appendix 2); this early start was noticed in most regions in The Netherlands (Table 6). Few breeding birds were in first-year plumage (1 out of 62 males, 9 out of 86 females), mostly in areas where some expansion is taking place or persecution is rife. The secondary sex ratio was with 54.8% in favour of males, as it is in most years (Table 6). Out of 45 specified nest failures, 27 were caused by human intervention (mostly intentionally); natural causes of failure included egg desertion (5x), egg predation (5x), nestling predation (4x), adverse weather (2x) and death of a parent (2x). Regional differences in prey choice, as depicted by prey remains found on and near nests, are summarised in Appendix 9. The age-composition of captured racing pigeons was elucidated from rings found on/near nests and in pellets: 1-calender-year (cy) old (35x), 2-cy old (20x), 3-cy old (7x), 4-cy old (3x), 5-cy old (1x), 6-cy old (2x) and 13-cy old (1x). Sparrowhawk Accipiter nisus: mean onset of laying showed quite pronounced regional differences (Table 8), but was on average rather late (1 May. range 12 April- 11 June. Appendix 2). Given the detailed reappraisal of how to back-calculate laying date by means of the age of nestlings (as quantified by standardised measurements) by Jan van Diermen and Hans Donkers (see elsewhere in this issue of De Takkeling), all laying dates so far published, including the ones calculated for 2001, should be retarded by 1.5 days. Among breeding males, 6.9% was in first-year plumage (29 sexed), among females 10.8% (83 sexed). Mean clutch size was 4.70 eggs (n=228. Appendix 3), mean brood size 3.88 (n=331, Appendix 4), Secondary sex ratio did not differ from equal: 477 males and 465 females (242 nests; Table 9). Among natural causes of nest failure, predation of eggs (9x) or nestlings (29x, mainly Goshawk) predominated. Common Buzzard Buteo buteo: breeding birds were mostly in adult plumage, i.e. 98.0% in males (n=201 aged) and 98.1% in females (n=262). Mean onset of laying was 4 April (9 March-4 June, n=935; Appendix 2), but with distinct regional differences (Table 10). Mean clutch size was 2.65 (n=484, Appendix 3), mean brood size 2.18 (n=1136, Appendix 4). In 259 nests, all nestlings were sexed. showing a preponderance of males (55,1% of 528 nestlings; Table 11). In 30 out 86 nest failures with know cause, humans were involved in 30 cases (mostly intentionally disturbing nests). Natural causes were egg desertion (15x), egg predation (1Ox), nestling predation (20x), adverse weather (1Ox) and loss of a parent (1x). The poor vole numbers were reflected in the small number of clutches with 5 eggs (n=2), neither of which resulted in complete fledging. Common voles were found in low numbers on nests in most regions (Appendix 10), although it should be taken into account that most nest visits were paid in the latter half of the nestling stage, i.e. the period that the chances of finding small mammals on nests are pretty small. The relatively high numbers of hares found as prey remain, as compared to rabbits, may indicate that young hares formed an important substitute for the dwindling rabbit numbers (caused by virus infections, principally VHS). Common Kestrel Falco tinnunculus: the majority of nests involved nest boxes (92% of 926 nests), with much smaller numbers in open crow’s nests (50x, in 5 cases in electricity towers) and cavities (8x). Kestrels nesting in nest boxes showed a slightly belter breeding success (89% of nests with at least fledgling) than those using open nests (80%). Mean onset of laying was 27 April (25 March-15 June, n=568; Appendix 2), mean clutch size 5.00 (n=469, Appendix 3) and mean brood size 4.28 (n=763, Appendix 4). Only 7 clutches with 7 eggs were recorded. Regional differences in breeding performance are depicted in Table 12. Hobby Falco subbuteo: the distribution is to a large extent complementary to the one in the 1970s; presently low densities prevail in woodland in the eastern half of the country and an increase in farmland habitats in the low-lying western half of the country. This is partly reflected in nest site choice (Table 15), with a preponderance of nests in poplars in most provinces and a high incidence of nesting on crow’s nests in electricity towers. This is quite different from the 1970s, when nesting in Scots pines predominated. Onset of laying was on average 10 June (30 May-13 July, n=57, i.e. by far the latest of all raptor species breeding in The Netherlands (Appendix 2), Mean clutch size was 2.94 (n=31. Appendix 3), mean brood size 2.36 (n=102. Appendix 4). Secondary sex ratio in 18 nests was 21 males and 25 females; overall sex ratio in 1996-2001 did not differ from unity (Table 14). Prey remains found near nests are summarised in Table 16, in which the scarcity of formerly important prey species like hirundines, skylark and sparrows is remarkable. Peregrine Falcon Falco peregrinus: 8 pairs were recorded in 2001. Since 1990, when Peregrines started to breed again in The Netherlands, the species has shown a steady increase from 1996 onwards (Table 18). All pairs breed in specially provided nestboxes on tall industrial structures. Seven out of 8 pairs were successful, raising 20 young (13 males, 7 females; Table 17). Since 1990, 88 young have fledged (Table 18). Onset of laying ranged between 23 February and 5 April, clutch size was 2x 3 and 5x 4 eggs; a clutch of 5 eggs was probably produced by two females and failed to hatch.