A total of 3112 nest record cards of raptors were submitted in 2017 (Appendix 1, handed in up to and including 5 January 2018), covering 15 species (including Milvus migrans, Haliaeetus albicilla, Circus macrourus and Pandion haliaetus, for which information from other sources was gleaned). The preceding winter was rather mild (frost index of 10.8 on a scale of 1-100). The summer was warm (summer index 72.6). Indices of abundance of prey groups were collected in several non-random regions. Common voles Microtus arvalis were present in moderate densities. Rabbit Oryctolagus cuniculus numbers remained low. Bird numbers in June-August were rather high. Social wasps were present in very low numbers despite an early start (late March queens building nests, then again early May after cold April) and initially high queen numbers dropping off to near-complete failure of nests by late June/early July. Honey-buzzard Pernis apivorus: onset of laying averaged 26 May (range 19 May-11 June). Clutch size was 9x 2 eggs, brood size 6x 1 and 13x 2 chicks. Despite the collapse in wasp numbers, Honey-buzzards were still able to feed their young with wasp broods (of which average comb diameter doubled between early July and early September, as based on combs found on nests of Honey-buzzards). Red Kite Milvus milvus: 11 breeding attempts were recorded, of which several were successful. Black Kite Milvus migrans: a single nest in the southern Netherlands, with two fledglings. White-tailed Eagle Haliaeetus albicilla: seven out of eleven pairs were successful and raised 11 chicks, a twelfth pair built a nest but refrained from egg laying. The increase in the population is now well under way (55 young raised in 2006-17). Marsh Harrier Circus aeruginosus: mean onset of laying was 24 April (range 1 April-3 June). Clutch size averaged 4.67 (n=76), mean number of fledglings per successful pair 3.22 (n=90). 63 complete broods with 208 nestlings were sexed: 51.7% male. Illegal nest destruction was recorded 17 times, nest predation 12 times. Food remains found on nests showed the importance of voles, but a wide array of birds and other small mammals were recorded as well (Appendix 5). Hen Harrier Circus cyaneus: three pairs nested on Texel, with mean lay date of 9 May (range 3-20 May), clutches of 3, 4 and 5 eggs and broods of 1, 2 and 3 chicks (1 male, 5 females). This species is in steep decline. Montagu’s Harrier Circus pygargus: no information as yet available for 2017 (but see www.werkgroepgrauwekiekendief.nl). Pallid Harrier Circus macrourus: a pair nested successfully in arable land in the province of Groningen, raising 4 chicks. The nest was protected against destruction from harvesting. Goshawk Accipiter gentilis: mean start of laying was 31 March (n=174, of which 52% started in March), clutch size averaged 3.42 eggs (of which 3x C/5 and 1x C/6 among 151 clutches), brood size (at ringing/fledging) 2.7 (253 nests). Sex ratio on 164 nests was in favour of males (52.7% males, on 444 chicks). Nest failures were human disturbance (7x), and natural (19x, of which three take-overs by Alopochen aegyptiaca and Anser anser. The prey list (n=807) predominently contained birds (95.5%), pigeons being numerically by far the most important (46% of all prey). Sparrowhawk Accipiter nisus: mean onset of laying was 29 April (range 11 April-31 May, n=125), with 58% of pairs starting in April. Clutch size averaged 4.88 (n=123), the number of fledglings per successful pair 4.09 (n=165). The secondary sex ratio among fledglings was slightly in favour of males (205 males, 175 females, 93 nests). Among nest failures, predation was important: 29x (24x nestlings, 5x adults), compare to 3x desertion, 5x adverse weather, and 3x human disturbance. Buzzard Buteo buteo: average start of egg laying was calculated at 5 April (range 17 March-10 May, n=407; 33% of all pairs started laying in March). Mean clutch size was 2.5 (n=370, with 16 C/4), mean brood size of successful pairs was 2.06 (n=659, with 11 B/4). Secondary sex ratio in 136 nests with 270 chicks was 58.5% males. Of 1075 nests closely followed, 22.6% failed to fledge chicks. Sex ratio varied with lay date: early clutches produced more females than later clutches. Known causes of nest failure included deliberate nest disturbance by humans (20x) and natural causes (49x). Diet was varied, with 41 bird species (39.2% of 868 prey items), 17 mammal species (59.2% of all prey items, lagomorphs, moles and voles being important in terms of biomass), and 10 snakes and amphibians. Osprey Pandion haliaetus: two successful nests in De Biesbosch each produced three fledglings (nests in tree and electricity pylon). Kestrel Falco tinnunculus: onset of laying averaged 24 April (range 27 March-14 June, n=474). Mean clutch size was 5.15 n=503), mean number of fledglings/successful nest 4.56 (n=618). Nest failures included human disturbance (6x) and natural causes (26x, of which 2x take-over by Egyptian Geese). Common voles were important among 1174 prey found in nestboxes and pellets (72%). A wide variety of birds (19 species) was also recorded as prey. Hobby Falco subbuteo: mean start of laying was 9 June (range 28 May-28 June, n=25). Mean clutch size was 2x 1, 1x 2 and 8x 3, mean number of fledglings/successful nest 9x 1, 29x 2, 23x 3 and 2x 4. Sex ratio in 6 nests was 5 males and 10 females. Of 97 nests with known outcome, 72 were successful. Causes of nest failure were predation (2x) and inclement weather (1x). Most pairs nested on old crow’s nests (62x Corvus corone, 2x Buteo buteo, 1x Corvus frugilegus). Many pairs nested on crow’s nests in electricity pylons (49.5%, out of 95 nests). Peregrine Falcon Falco peregrinus: lay date averaged 20 March (range 2 March-13 April, n=36). Clutch size was 3x 2, 4x 3, 17x 4 and 1x 5 eggs, brood size averaged 2.68 (n=47). Secondary sex ratio on 32 nests was 39 males and 47 females. An extensive prey list from breeding sites in the province of Zuid-Holland showed a preponderance of racing/feral pigeons (117 out of 168 prey items).

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Werkgroep Roofvogels Nederland

R.G. Bijlsma. (2018). Trends en broedresultaten van roofvogels in Nederland in 2017. De Takkeling, 26(1), 5–47.