Trends en broedresultaten van roofvogels in Nederland in 2014
De Takkeling , Volume 23 - Issue 1 p. 4- 51
A total of 2830 nest record cards of raptors were submitted in 2014 (Appendix 1, handed in up to and including 10 January 2015), covering 12 species. The preceding winter was extremely mild (frost index of 1.4 on a scale of 1-100). The summer was very warm (summer index 79.0). Voles (Microtus arvalis and Myodes glareolus) and mice (Apodemus sylvaticus) showed peak numbers and outbreaks throughout the country. Indices of Rabbit Oryctolagus cuniculus on average were poor, but bird numbers were high (early start of laying in many species, good reproduction with second and third broods in many species). Social wasps were present in very poor numbers, with an early emergence of queens (on average 29 March, n=10, sd=14.1) and significant losses in June. Few wasps (mostly Common Wasp Vespula vulgaris) succeeded in raising large colonies that were active well into autumn. Over the years, emergence dates of queens have advanced by more than a month between 1975 and 2014 (Fig. 9), especially after the 1990s. In parallel with this trend, and over the same period, Honey buzzards have advanced their lay dates, to the extent that in 2014 77% of 31 pairs started laying in May. Honey Buzzard Pernis apivorus: onset of laying averaged 26 May (sd=8.21, n=31, range 11 May-13 June). Clutch size was 1x 1 and 10x 2 egg(s). Mean brood size was 3x 1 and 33x 2 chick(s). Chick mortality was low despited generally low wasp availability. Most causes of failure involved predation (4x chicks, 5x adults). A wide range of tree species was used for nesting (most nests built by Honey Buzzards, i.e. 23 of 28), with an average nest height of 17.0 m (n=34, sd=6.6, range 10-35 m). Prey remains found at one nest in the northern Netherlands mainly consisted of wasp combs: 8x V. vulgaris, 2x V. rufa, 29x V. germanica/rufa, 2x Vespa crabro, 1x Columba palumbus squab, 1x Erithacus rubecula and 6x Rana temporaria. Red Kite Milvus milvus: a successful breeding attempt (3 chicks fledged) was recorded in the eastern Netherlands, a failed attempt (in early chick stage) in the southern Netherlands. Black Kite Milvus migrans: three nesting attempts in the southern Netherlands (2x Limburg, 1x Noord-Brabant) raised 2x 2 and 1x 3 chicks. Another pair in the central Netherlands failed, presumably in the early chick stage. White-tailed Eagle Haliaeetus albicilla: of five pairs, three succeeded in raising 2x 2 (both in Biesbosch) and 1x 3 chicks (Oostvaardersplassen). The nests in Lauwersmeer and Roggebotzand failed during incubation. Marsh Harrier Circus aeruginosus: mean onset of laying was 25 April (sd=13.2, n=115, range 3 April-2 June), i.e. much earlier than usual (Appendix 2). Clutch size averaged 4.56 (sd=0.97, n=87, range 2-7; Appendix 3), mean number of fledglings/successful pair 3.31 (sd=1.00, n=127, range 1-5; Appendix 4). 80 complete broods were sexed: 141 males and 121 females. Over the years 1996-2013, a biased secondary sex ratio has been prevalent (Table 3: 53.1% males in 1454 nests with 4633 nestlings). Illegal nest destruction was recorded seven times, nest predation by Red Fox Vulpes vulpes four times (2x eggs, 2x nestlings); nest desertion was recorded once. Food remains found on nests showed 46.8% birds, 52.8% mammals and 0.3% amphibians (n=271); voles (including unidentified rodents) were a prominent prey species in 2014 (37% in numbers; Appendix 5). Hen Harrier Circus cyaneus: the population continues to decline. Texel is presently the only Wadden Sea Island with breeding pairs (4 nests and 1 territory). On the mainland, two pairs were registered in farmland in Groningen (1x 4 chicks). Montagu’s Harrier Circus pygargus: the Dutch population amounted to 47 pairs, raising 117 young (in 39 successful – often protected - nests). Goshawk Accipiter gentilis: mean start of laying was 29 March (63% of 168 clutches started in March; range 12 March-19 April, Appendix 2), clutch size averaged 3.58 eggs (sd=0.66, n=139, range 1-5; Appendix 3), brood size (at fledging) 2.67 (Appendix 4, sd=0.80, n=275, range 1-5). Over the years (1996-2014), the proportion of pairs with clutches of 4 eggs showed wide annual variations without clear trend, whereas the proportion of pairs producing broods of four has declined (Fig. 10). Secondary sex ratio in 143 nests was in favour of males (55.2%), typical for the Dutch population at large (mean for 1996-2014 55.1% male in 4659 nests with 12,410 nestlings). Among identified causes of nest failure in The Netherlands, illegal human activities were important (7 out of 17). Food remains found on and near nests showed a preponderance of pigeons (mostly Racing Pigeon Columba livia and Woodpigeon C. palumbus; 41.4% of 673 prey remains; Appendix 6). Other important prey groups (in numbers) were: corvids including Jay Garrulus glandarius (19.6%), thrushes (9.7%) and Starling Sturnus vulgaris (5.9%). Raptors as prey were present in five species (2.4% of all prey remains), owls in four species (n=9, i.e. 1.3%). Sparrowhawk Accipiter nisus: mean onset of laying was 26 April (range 8 April-15 May, sd=7.26, n=115; Appendix 2); the proportion of pairs starting in April was 81%, indicating favourable conditions in spring. Clutch size averaged 5.22 (sd=0.78, n=98, Appendix 3), the number of fledglings per successful pair 4.16 (sd=1.25, n=157, Appendix 4). Among age-identified breeding birds, the proportion of first-years was very high, i.e. 34% in males (n=35) and 36% in females (n=53). The secondary sex ratio among fledglings was slightly in favour of males (52.1% males, 359 young, 86 nests). The long-term sex ratio was 51.2% male (1996=2014: 12,690 chicks on 3332 nests). Among nest failures, predation was most often recorded: 28x among 31 cases of natural losses (11x eggs, 13x chicks, 4x adults). Buzzard Buteo buteo: average start of egg laying was calculated at 31 March (range 14 March-7 May, sd=8.16, n=398; 56% of all pairs started laying in March; Appendix 2), earlier than ever recorded since 1986 (Fig. 13) and indicative of high vole abundance and very mild preceding winter. Mean start of laying showed wide fluctuations between 1984 and 2014 (no clear trend), with the earliest onsets being rather fixed at 15 March and dates of latest onsets advancing (at present second half of April, rather than first half of May) (Fig. 12). Mean clutch size was 2.83 (sd=0.58, n=246, with 20 C/4), mean brood size of successful pairs was 2.25 (sd=0.77, n=715; Appendix 4). For the province of Drenthe, in 1990-2014 a steady decline – with ups and downs - in the number of broods with 4 eggs or 4 chicks was recorded. The vole outbreak in 2014 resulted in a high proportion of broods with 4 chicks, but not nearly so high as recorded in the 1990s. Deliberate nest disturbance was recorded in 14 out of 36 cases with known cause of failure. A single nest on the ground failed during the early chick stage (inclement weather). Secondary sex ratio in 128 nests was male-biased: 153 males, 136 females (as in the longer run, i.e. 1996-2014: 53.7% for 7150 young on 3761 nests). Food choice was varied, with 28 bird species (28.6% of 771 prey items), 14 mammal species (69.4% of all prey items, voles most important with 33.8% in numbers), and some snakes, frogs and toads (Appendix 7). Osprey Pandion haliaetus: a pair in the Biesbosch built two nests on electricity pylons, but no eggs were laid. Eurasian Kestrel Falco tinnunculus: onset of laying averaged 13 April (range 11 March-14 June, n=476; Appendix 2), almost a month earlier than in 2013 (when extremely late). This early start was recorded throughout the country. Mean clutch size was 5.47 (sd=0.80, n=445; Appendix 3), mean number of fledglings/successful nest 4.75 (sd=1.36, n=581, Appendix 4). Nest failures included human disturbance (1x) and natural causes (11x). Prey remains in nestboxes consisted mostly of voles (84% of 173 prey remains), the rest mostly birds (Table 12). Hobby Falco subbuteo: mean start of laying was 8 June (range 30 May-30 June, n=20; Appendix 2). Mean clutch size was 2x 2 and 4x 3, mean number of fledglings/successful nest 2.21 (sd=0.83, n=43 Appendix 4). Sex ratio in 7 nests was 7 males and 9 females. Of 61 nests with known outcome, 53 were successful (biased in favour of successful nests, which are more easily recorded). Most pairs nested on old crow’s nests (44 of Corvus corone, out of 45 nests). Mean nest height excluding electricity pylons was 17.1 m (sd=6.5, n=33). The proportion of pairs nesting on crow’s nests in electricity pylons was 26.9% (n=52 nests). Peregrine Falcon Falco peregrinus: lay date averaged 15 March (range 26 February-4 April, sd=9.19, n=24). Clutch size was 3x 3 and 17x 4, brood size averaged 2.66 (sd=0.61, range 2-4, n=29). Secondary sex ratio was in favour of males: 35 males and 27 females on 23 nests. A nest on the ground (in the dunes of Schiermonnikoog) with four eggs failed. Extensive prey lists from four breeding sites showed a preponderance of pigeons (mostly Feral and Racing), i.e. 39-75% of all prey.
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