Trends en broedresultaten van roofvogels in Nederland in 2008

A total of 3274 nest cards of raptors were submitted in 2008 (Appendix 1, up to and including 31 December 2008), covering 12 species. The preceding winter was mild (frost index of 5.9 on a scale of 1-100), the spring rather cool but improving in April through mid-May, then becoming wetter. The summer was warm (summer index 70.0, on a scale of 1 -100). Food supply varied from rather poor (voles, mice) to poor (rabbit, hare), normal (birds) and rather poor (social wasps, after a promising start in early spring, with German and Common wasp Vespula germanica and V. vulgaris showing heavy mortality among queen nests in late May, but surviving nests remained active up to September-November). Local variations in food supply were striking, especially regarding Common Voles (abundant in Friesland, a low in Zeeland), with concomitant variations in the breeding performance of vole-eaters. Honey-buzzard Pernis apivorus: onset of laying averaged 23 May (range 16 May-16 June), with a single peak in 16-30 May. Only clutches with 2 eggs were recorded (N=14), and mean brood size was 1.7 (SD=0.45, N=24). Food items on nests consisted mostly of wasp combs (90), with a relatively high frequency of vertebrate prey (10 frogs, 1 small Grass Snake Natrix natrix, 4 birds in 2 species, and 1 bumblebee nest). Out of 25 nests, 23 were built by Honey-buzzards, the other nests were made by Buzzard (1) and an unknown raptor (1). Mean nest height was 15.0 m (SD=5.0, N=22). In the central Netherlands, adult Honey-buzzards were captured and fitted with radio tags or transmitters. The results of this research will be published shortly. Red Kite Milvus milvus: a nest in the NE Netherlands fledged a single young. This was – as far as is known – the first nesting attempt in The Netherlands since 1988. White-tailed Eagle Haliaeetus albicilla: the pair in the Oostvaardersplassen built a new nest and raised two chicks (male and female). This is the third consecutive breeding attempt here. The first egg was laid on 1 March. Both chicks were (colour)-ringed. Food consisted mainly of goslings, ducks. Coots, Musk Rats and Carps. Marsh Harrier Circus aeruginosus: mean onset of laying was 22 April (range 5 April-14 May; Appendix 2, Table 3). Clutch size averaged 4.64 (SD=0.91, N=55; Appendix 3), mean number of fledglings/successful pair 3.16 (SD=I .09 N=67, Appendix 4). All surviving nestlings in 55 nests were sexed: 90 males and 87 females. Over the years 1996-2008, a biased secondary sex ratio has been prevalent (Table 4: 52.8% males in 1141 nests with 3358 nestlings). Illegal nest destruction was widespread in the province of Friesland. Hen Harrier Circus cyaneus: the Dutch population again declined, a consistent trend in the past decades, from 46 pairs in 2007 to 32 in 2008 (30 on Wadden Sea Islands, 2 in Oostvaardersplassen). Mean start of laying averaged 30 April (range 15 April-22 May; Appendix 2), clutch size 4.4 eggs (range 1-6; Appendix 3), brood size of successful nests 3.2 (range 1-6; Appendix 4). Montagu’s Harrier Circus pygargus: see www.grauwekiekendief.nl for details of satellite tracking. The 2008 breeding season will be covered in the next Takkeling. The number of Dutch breeding pairs in 2008 was 48, as in 2007. Goshawk Accipiter gentilis: mean start of laying was early in 2008 (30 March, 53.2% of 235 clutches started in March, range 9 March-24 April, Appendix 2), clutch size averaged 3.30 eggs (SD=0.77, N=194, range 1-5; Appendix 3), brood size (at fledging) 2.63 (Appendix 4, SD=0.86, N=312, range 1-5). Secondary sex ratio on 224 nests was in favour of males (54.1%), and in line with the long-term mean for 1996- 2008 (56.4% male in 3514 nests with 9457 nestlings). Among identified causes of nest failure, illegal human activities predominated (11 out of 17), with natural causes being desertion (4x), egg predation (1x), and chick predation (1x). Food remains found on and near nests showed a preponderance of pigeons (mostly Racing Pigeon Columba livia and Woodpigeon C. palumbus; 42.6% of 1088 prey remains). Among age-identified Racing Pigeons, first-year birds were most often recorded (45.6% of 57 rings found on nests and in pellets). Other important prey groups (in numbers) were; corvids including Jay Garrulus glandarius (25.7%), Starling Sturnus vulgaris (5.1%), thrushes (6.8%) and woodpeckers (4.3%). Sparrowhawk Accipiter nisus: mean onset of laying was 30 April (range 11 April-27 May, N=161; Appendix 2); the proportion of pairs starting in April was 59% (compared to 76% in 2007). Mean clutch size was 4.75 (N=104, Appendix 3), mean number of fledglings/successful pair 4.02 (N=161). One nest fledged 7 chicks, and only 7 nests fledged 6 chicks (Appendix 4). The secondary sex ratio was fifty-fifty (289 males, 280 females, 143 nests). Among the breeding birds, 15.9% of the males (N=44) was first-year, and 16.1% of the females (N=62). Buzzard Buteo buteo: average start of egg laying was calculated at 5 April (range 12 March-21 May, N=562; Appendix 2). Mean clutch size was 2.57 (N=382, with only 16 C/4), mean brood size of successful pairs was 2.06 (N=842; Appendix 4). The secondary sex ratio in 188 nests was male-biased; 54.4% of 393 young. Food choice was varied, with 37 bird species, 15 mammal species, 2 snakes species, 3 frog species and 2 fish species (Appendix 10, N=780). Out of 77 identified causes of failure, 46 could be attributed to deliberate human action (mostly in the province of Friesland). Natural causes of failure included desertion (2x), egg predation (1 lx), chick predation (9x), death of parent (lx), adverse weather (4x) and take-over/disturbance by Egyptian Geese Alopochen aegyptiaca (4x). Osprey Pandion haliaetus: nesting attempts, in the Oostvaardersplassen or elsewhere, were not recorded. Eurasian Kestrel Falco tinnunculus: onset of laying averaged 21 April (range 13 March-7 June, N=651; Appendix 2). Mean clutch size was 5.2 (N=492; Appendix 3), mean number of fledglings/successful nest 4.3 (N=711, Appendix 4). Very large regional variations in reproductive output were recorded, being high in the northern Netherlands and low in the SW Netherlands; prey remains found in nest boxes indicated that vole numbers were particularly low in the SW Netherlands. Out of 923 registered breeding sites, 899 were in nest boxes. Causes of failure were mostly natural, i.e. desertion and predation. Hobby Falco subbuteo: has declined substantially in the past decades, with clear indications of widespread reproductive failure. The proportion of pairs nesting on crow’s nests in electricity pylons in 2008 was 23.7% (N=72 nests). Mean start of laying was 6 June (range 30 May-17 June, N=22; Appendix 2). Mean clutch size was 2.6 (N=7; Appendix 3), mean number of fledglings/successful nest 2.3 (N=38; Appendix 4). Secondary sex ratio in 5 nests was in favour of females: 6 males and 9 females (sexes identified by body mass and vocalisations). Peregrine Falcon Falco peregrinus: details will be published by the Peregrine Working Group.