Trends en broedresultaten van roofvogels in Nederland in 2007

A total of 3891 nest cards of raptors were submitted in 2007 (Appendix 1, up to and including 17 January 2008), covering 12 species. The preceding winter was extremely mild (frost index of 0.9 on a scale of 1-100, a value never reached before since 1706), spring extremely dry and warm through early May. The summer was rather cool with heavy downpours (summer index 62.2, on a scale of 1-100). Food supply varied from very good (voles, mice) to very poor (rabbit, hare), good and early (birds) and poor (social wasps, after a promising start in early spring, with German and Common wasp Vespula germanica and V. vulgaris showing heavy mortality among embryo nests in May, but surviving nests active through September-October). European Honey-buzzard Pernis apivorus: onset of laying averaged 26 May (range 16 May-13 June), with a bimodal distribution peaking in 20-25 May and 31 May-4 June. Only clutches with 2 eggs were recorded (N=l I), and mean brood size was 1.7 (SD=0.44, N=27). Food items on nests consisted mostly of wasp combs (80), with a relatively high frequency of vertebrate prey (13 frogs, 6 birds in 4 species) and 1 bumblebee nest. Out of 32 nests, 28 were built by Honey-buzzards, the other nests were made by Buzzard (2), Goshawk (1), Carrion Crow (1) and man (1). Mean nest height was 15.6 m (SD=4.14, N=28). Black Kite Milvus migrans: a nesting attempt was recorded for the western Netherlands (nest built in poplar, but presumably no egg laying; de Barse et al. 2007). This fits the trend of the past years, when several more attempts were registered. White-tailed Eagle Haliaeetus albicilla: a successful breeding attempt was recorded in the Oostvaardersplassen, the second for The Netherlands by the same pair on the same nest as in 2006 (female colour-ringed as nestling in 2003 in northern Germany; adult male not banded). Activities at the nest were recorded with a webcam. A single chick fledged on 8 July (when 72 days old), the egg was laid on 9 March and hatched on 15 April. The female chick was ringed on 24 May (sex confirmed by DNA analysis). Prey mainly consisted of geese, ducks, coots and fishes. Marsh Harrier Circus aeruginosus: mean onset of laying was 27 April (range 2 April-10 June; Appendix 2, Table 4). Clutch size averaged 4.25 (SD=0.83, N=69; Appendix 3), mean number of fledglings/successful pair 3.34 (SD=1.20, N=120; Appendix 4). All surviving nestlings in 73 nests were sexed: 129 males and 111 females. Over the years 1996-2007, a biased secondary sex ratio has been prevalent (Table 5: 52.8% males in 1086 nests with 3184 nestlings). Illegal nest destruction was again practiced, mostly in the province of Friesland. Hen Harrier Circus cyaneus: the decline of the Dutch population continues unabated, with only 46 pairs registered in 2007 (44 on the Wadden Sea Islands, 2 in Oostvaardersplassen). Mean start of laying averaged 1 May (range 16 April-25 May; Appendix 2), clutch size 4.45 eggs (range 3-5; Appendix 3), brood size of successful nests 2.14 (range 1-4; Appendix 4). Out of 35 closely observed nests on the Wadden Sea Islands, only 18 succeeded in raising at least one fledglings; both nests on the mainland failed. Poor food supply is indicated as a major problem, probably exacerbated by habitat changes. Montagu’s Harrier Circus pygargus: see www.grauwekiekendief.nl for details of satellite tracking. Next issue of De Takkeling wil present an overview of the 2007 breeding season. Goshawk Accipiter gentilis: 2007 on average showed an early start of laying (30 March, 60% of 279 clutches started in March, range 13 March-28 April), a large clutch size of 3.33 eggs (SD=0.98, N=242, range 1-5; Appendix 3), and a large brood size (at fledging) of 2.72 (Appendix 4, SD=0.89, N=270, range 1-4). Secondary sex ratio on 262 nests was in favour of males (54.6%), and in line with the long-term mean for 1996-2007 (55.3% male in 3290 nests with 8867 nestlings). Among identified causes of nest failure, illegal human activities predominated (18 out of 30), with natural causes being desertion (1x), egg predation (2x), chick predation (1x), adverse weather (3x), death of parent (3x) and take-over/disturbance by Egyptian Geese Alopochen aegyptiacus (2x). The long-running series from Drenthe shows a clear correlation between start of laying and mean March temperature, high temperatures accompanied by an earlier start. Consistent regional variations in start of laying and other breeding parameters are probably based on variations in food supply. In this regard, the early start and good breeding performance in 2007 was consistent with a warm spring and high food supply. Food remains found on and near nests showed a preponderance of pigeons (mostly Racing Pigeon Columba livia and Woodpigeon C. palumbus; 43.8% of 1701 prey remains). Among age-identified Racing Pigeons, first-year birds were most often recorded (51.6% of 91 rings found on nests and in pellets). Other important prey groups (in numbers) were: corvids including Jay Garrulus glandarius (17.8%), Starling Sturnus vulgaris (8.2%), thrushes (8.1%) and woodpeckers (4.5%). Sparrowhawk Accipiter nisus: mean onset of laying was 27 April (range 13 April-4 June, N=210; Appendix 2), an early start – in conjunction with a warm and dry spring – that was recorded throughout The Netherlands (Table 9). Mean clutch size was 4.99 (N=192, Appendix 3), mean number of fledglings/successful pair 4.04 (N=263, Appendix 4). The secondary sex ratio was fifty-fifty (290 males, 294 females, 190 nests). Among the breeding birds, first-years were well represented: 20% in males and 22% in females. Conditions for settling in 2007 were favourable, i.e. warm and dry spring weather and good food supply. Pairs consisting of first-years showed a later start of laying in comparison with adult pairs (14 May versus 22 April), a smaller clutch size (respectively 4.3 and 5.3 eggs) and a smaller brood size at fledging (respectively 4.1 and 4.4 young). Buzzard Buteo buteo: average start of egg laying was rather early, i.e. on 4 April (range 14 March-2 May, N=714; Appendix 2). Mean clutch size was 2.62 (N=530, with many C/4), mean brood size of successful pairs was 2.12 (N=1060; Appendix 4). Early start and large clutch and brood sizes are consistent with high vole numbers (as found in vole transects, and recorded throughout The Netherlands). The secondary sex ratio in 240 nests was male-biased: 57.5% of 492 young. Food choice was varied, with 58 bird species, 16 mammal species, 7 species of reptiles and amphibians, and two fishes (Appendix 10, N=1319). Common voles Microtus arvalis are probably underrecorded (24% in number), because most nests are visited in the later nestling stage only (when chicks can be ringed, and few voles remain uneaten on the nest). Out of 108 identified causes of failure, 72 could be attributed to deliberate human action (mostly in the province of Friesland). Natural causes of failure included desertion (7x), egg predation (13x), chick predation (3x), death of parent (1x), adverse weather (7x) and take-over/disturbance by Egyptian Geese (7x). Osprey Pandion haliaetus: nesting attempts, in the Oostvaardersplassen or elsewhere, were not recorded (Frank de Roder). Eurasian Kestrel Falco tinnunculus: onset of laying averaged 16 April (range 12 March-1 June, N=536; Appendix 2). Mean clutch size was 5.46 (N=498; Appendix 3), mean number of fledglings/successful nest 4.76 (N=718, Appendix 4). Early starts and high clutch and brood sizes were recorded throughout The Netherlands. Out of 850 registered breeding sites, 822 were nest boxes. Causes of failure were mostly natural, i.e. desertion and predation. Eurasian Hobby Falco subbuteo: most nests were located in Friesland and Noord- Brabant, very few in extensive woodland in the eastern Netherlands (where the species used to be common). Present numbers appear to be stable at a much lower density than in the 1980s and 1990s. The proportion of pairs nesting on crow’s nests in electricity pylons in 2007 was 25% (N=82 nests). Mean start of laying was 8 June (range 29 May-21 June, N=34; Appendix 2). Mean clutch size was 3.1 (N=18; Appendix 3), mean number of fledglings/successful nest 2.22 (N=65; Appendix 4). Secondary sex ratio in 14 nests was in favour of females: 14 males and 19 females (sexes identified by body mass and vocalisations). Peregrine Falcon Falco peregrinus: almost 40 pairs were registered in 2007. Details will be published by the Peregrine Working Group. Trends in raptor populations: wide, but consistent, variations in various parts of The Netherlands. The well-wooded eastern Netherlands, traditionally the bulwark of tree-nesting species, has seen a long-term decline in Kestrel and Hobby since the mid-1970s, and stable populations of Buzzard, Sparrowhawk, Goshawk and Honey-buzzard throughout the 1980s and early 1990s. Since then, most species except Buzzard have been in decline (Figs. 9-12). A very different trend prevailed in the western and northern Netherlands, where woodland is scarce and farmland predominates. Tree-nesting species have gradually colonised these regions from the 1980s onwards (Wadden Sea Islands and province of Zeeland even later, i.e. since 1990s), and became quite common wherever nesting opportunities are available. Simultaneously, Kestrels declined whereas harriers either disappeared (or nearly so: respectively Montagu’s Harrier and Hen Harrier) or started to decline (Marsh Harrier). These trends signify a substantial extension of the breeding distribution of treenesting species in The Netherlands concurrent with a decline of the same species in strongholds in the eastern part of the country, a large decline of the Kestrel throughout The Netherlands, a stabilisation of increases of tree-nesing species in the western Netherlands in the 2000s, the near-demise of Hen Harriers as a breeding bird, and a slight decline in Marsh Harrier.