A total of 3442 nest record cards of raptors were submitted in 2015 (Appendix 1, handed in up to and including 10 March 2016), covering 11 species. The preceding winter was mild (frost index of 6.8 on a scale of 1-100). The summer was warm (summer index 67.2). Indices of prey groups were collected in several non-random parts of the country. Voles (Microtus arvalis and Myodes glareolus) and mice (Apodemus sylvaticus) were down in numbers after the steep peak of 2014, except in Friesland where high numbers prevailed during much of the spring and summer in the peat and clay districts. In 1968-2015, Microtus arvalis numbers were monitored via pellets of Long-eared Owls Asio otus, separately for summer and winter. Peak numbers have been smaller in recent decades, but 2014 showed a high peak, extending into the winter of 2014/15 but crashing in the summer of 2015 (for the Veluwe in the central Netherlands). Also, Rabbit Oryctolagus cuniculus numbers were low, despite a very small increase in two indices for Drenthe and Veluwe. Bird numbers in June-August were high, as in 2014, perhaps indicative of good reproduction in the preceding months. Social wasps were present in moderate numbers (locally higher, though), with a rather late emergence of queens (on average 7 May, n=10) and significant losses in June (8 out of 10 Dolichovespula saxonica nests failed during the embryo stage). The average peak in traffic rate was recorded on 25 July, with worker activity in most nests ending by early August (few nests continued into September, a single Vespula vulgaris even into November). Honey Buzzard Pernis apivorus: onset of laying averaged 24 May (sd=4.5, n=17, range 19 May-3 June). Clutch size was 11x 2 eggs. Mean brood size was 9x 1 and 19x 2 chick(s). A wide range of tree species was used for nesting (most nests built by Honey Buzzards, i.e. 14 of 21), with an average nest height of 15.7 m (n=18, sd=8.5, range 8.5-25 m). Prey remains found on nests in several parts of The Netherlands mostly consisted of wasp combs: 38x V. vulgaris, 6x V. rufa, 21x V. germanica, 27x Vespula sp. and 6x Vespa crabro. Other prey included 1x Turdus merula, 1x Erithacus rubecula and 1x Fringilla coelebs. Red Kite Milvus milvus: 8 breeding attempts were recorded, of which at least four were successful in raising 1-2 chicks to fledging. Black Kite Milvus migrans: of four nesting attempts in the southern Netherlands, two were successful. White-tailed Eagle Haliaeetus albicilla: of six pairs, five succeeded in raising 1x 1 (Zwarte Meer), 3x 2 (IJsselmonding, Oostvaardersplassen, Dordtse Biesbosch) and 1x 3 chicks (Brabantse Biesbosch). The nest in Lauwersmeer failed during incubation. A seventh pair near Zuidlaardermeer did not yet built a nest. Marsh Harrier Circus aeruginosus: mean onset of laying was 28 April (sd=11.4, n=81, range 9 April-14 June). Clutch size averaged 4.61 (sd=5.8, n=76, range 2-7), mean number of fledglings/successful pair 3.40 (sd=1.0, n=95, range 1-5). 64 complete broods were sexed: 124 males and 95 females. Over the years 1996-2015, a biased secondary sex ratio has been prevalent (53.3% males in 1518 nests with 4852 nestlings). Illegal nest destruction was recorded six times, nest predation nine times (5x eggs, 3x nestlings, 1x parent). Food remains found on nests showed 52.8% birds (in 13 species) and 52.8% mammals in 7 species (n=193); lagomorphs constituted 24% in numbers. Hen Harrier Circus cyaneus: the population continues to decline. On the Wadden Sea Islands, Texel held 3-4 pairs, Terschelling 2. On the mainland, pairs were found in Groningen (3 of which successful) and Flevoland (1, successful). Montagu’s Harrier Circus pygargus: the Dutch population amounted to 44 pairs, raising 112 young by 33 successful pairs. Goshawk Accipiter gentilis: mean start of laying was 2 April (range 18 March-21 April, n=139), clutch size averaged 3.21 eggs (sd=0.73, n=128, range 1-5), brood size (at ringing/fledging) 2.60 (sd=0.83, n=198, range 1-4). The distribution of clutch size has shifted over the years: before 1992 (based on Bijlsma 1993) C/3 and C/4 accounted for resp. 37% and 49% of 273 clutches, compared to resp. 49% and 36% of 128 clutches in 2015. At the same time, C/2 has increased, with 18% before 1992 and already 33% of all completed clutches in 2015. Secondary sex ratio in 133 nests was strongly in favour of males (59.5%). Among identified causes of nest failure in The Netherlands, illegal human activities were important (7 out of 23), with natural causes including egg desertion (4x), predation (7x), weather (3x) and take-overs by Egyption Geese (2x). Food remains found on and near nests showed a preponderance of pigeons (mostly Racing Pigeon Columba livia and Woodpigeon C. palumbus; 40.8% of 661 prey remains). Other important prey groups (in numbers) were: corvids (20.0%), thrushes (9.2%) and Starling Sturnus vulgaris (4.2%). Mammals constituted 3.2% of all prey items found on/near nests. Sparrowhawk Accipiter nisus: mean onset of laying was 1 May (range 14 April-26 May, n=79), with 52% of pairs starting in April. Clutch size averaged 4.98 (sd=0.82, n=76, including 3x C/7), the number of fledglings per successful pair 3.97 (sd=1.20, n=84). The secondary sex ratio among fledglings was almost fifty-fifty (48.8% males, 267 young, 70 nests). Among nest failures, predation was important: 17x eggs, 21x chicks, 2x adults. Inclement weather accounted for one failure. Buzzard Buteo buteo: average start of egg laying was calculated at 4 April (range 16 March-23 May, sd=7.7, n=498; 32% of all pairs started laying in March, i.e. much less than in the vole year of 2014). Mean clutch size was 2.53 (sd=0.72, n=415, with 31 C/4 and 1 C/5), mean brood size of successful pairs was 1.96 (sd=0.72, n=751, with only 6 B/4). Secondary sex ratio in 162 nests with 316 chicks was fifty-fifty (50.3% males). Of 1313 nests closely followed, 311 failed to fledge chicks (23.7%). Known causes of nest failure included deliberate nest disturbance by humans (26 out of 106 nests), and natural causes such as egg predation (3x), chick predation (15x), adult predation (2x), adverse weather (28x) and take-overs by Egyptian Geese (12x). Food choice was varied, with 47 bird species (37.3% of 1118 prey items), 16 mammal species (59.6% of all prey items, lagomorphs and moles being important in terms of biomass, Microtus arvalis, Columba palumbus also in terms of numbers), some snakes, frogs and toads and one Abramis brama. Osprey Pandion haliaetus: nest building was recorded in the Biesbosch by two pairs, but again (as in 2014) no eggs were laid. Kestrel Falco tinnunculus: onset of laying averaged 17 April (range 14 March-18 June, n=622), locally even as early as 11 April (in Friesland, where the vole peak of 2014 continued well into 2015). Mean clutch size was 5.36 (sd=0.93, n=625), mean number of fledglings/successful nest 4.64 (sd=1.29, n=745). In the province of Friesland, at least 7 presumed second clutches were recorded. Nest failures included human disturbance (2x) and natural causes (27x, of which predation 23x, adverse weather 1x and take-over by Egyptian Geese 2x). Voles were still important among 862 prey found in nestboxes (50%), especially in Friesland, the only region where the vole peak of 2014 extended well into 2015 (96% voles among 85 prey). A wide variety of birds (21 species) was also recorded as prey. Hobby Falco subbuteo: mean start of laying was 8 June (range 29 May-20 June, n=15). Mean clutch size was 1x 2 and 9x 3, mean number of fledglings/successful nest 4x 1, 22x 2 and 19x 3. Sex ratio in 3 nests was 2 males and 3 females. Of 71 nests with known outcome, 55 were successful. Most pairs nested on old crow’s nests (47 of Corvus corone, 2 of Buteo buteo). Mean nest height excluding electricity pylons was 15.7 m (n=33). The proportion of pairs nesting on crow’s nests in electricity pylons was 24% (n=69 nests). Peregrine Falcon Falco peregrinus: lay date averaged 18 March (range 2 March-15 April, sd=9.3, n=19). Clutch size was 6x 3, 7x 4 and 1x 5 eggs, brood size averaged 2.81 (sd=0.90, range 1-4, n=26). Secondary sex ratio on 20 nests was 32 males and 23 females. Extensive prey lists from three breeding sites in Zeeland showed a preponderance of pigeons (31.4%, mostly Feral and Racing), waders (26.1%) and Sturnus vulgaris (14.1%) (n=283 prey).

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Werkgroep Roofvogels Nederland

R.G. Bijlsma. (2016). Trends en broedresultaten van roofvogels in Nederland in 2015. De Takkeling, 24(1), 5–60.