Trends en broedresultaten van roofvogels in Nederland in 2013
De Takkeling , Volume 22 - Issue 1 p. 4- 54
A total of 2886 nest record cards of raptors were submitted in 2013 (Appendix 1, handed in up to and including 26 January 2013), covering 12 species. The preceding winter was normal (frost index of 23.3 on a scale of 1-100), but with with several bouts of frost and extending into a prolonged cool spring (through June). The summer was warm (summer index 72.2), but with below-average temperatures up to and including June. Vole (Microtus arvalis and Myodes glareolus) and mice (Apodemus sylvaticus) abundances were very poor throughout The Netherlands. Indices of Rabbit Oryctolagus cuniculus were again poor, as were bird numbers (late start, poor reproduction). Social wasps were present in poor numbers, with on average a late start and an early ending of colonial activities (German and Common Wasp Vespula germanica and V. vulgaris, Saxon Wasp Dolichovespula saxonica). Honey Buzzard Pernis apivorus: onset of laying averaged 28 May (range 18 May-25 June), with an extended laying peak from mid-May to early June (10 out of 49 clutches started in June). One of the late laying pairs (11 June) had been loggered in 2012 (when breeding had started on 26 May, the male of this pair had wintered in Ghana). Clutch size was 1x 1 and 22x 2 egg(s). Mean brood size was 13x 1 and 34x 2 chick(s). Mortality among chicks was quite common because of poor wasp supply. Of 70 nests recorded, eggs were laid in 62 nests, of which 14 failed. Most causes of failure involved predation. A wide range of tree species was used for nesting (most nests built by Honey Buzzards, i.e. 42 of 36), with an average nest height of 16.7 m (N=37, SD=5.9, range 9.5-35.0 m). Prey remains found on nests were mainly wasp combs: 42x V. vulgaris, 6x V. germanica, 7x V. rufa, 24x V. germanica rufa, 14x Vespa crabro, 4x Columba palumbus squab, 1x chick of Pernis apivorus, 1x Turdus merula, 1x thrush Turdus sp., 1x Rana temporaria and 1x Pelophylax spp. Frogs are underrecorded, given the frequency with which a male was seen providing frogs (which were not recorded when the nest was visited). Red Kite Milvus milvus: apparently one failed breeding attempt in the eastern Netherlands. Black Kite Milvus migrans: a nesting attempt took place in the southern Netherlands (Limburg) where two chick fledged (start of egg laying 3 May). Another pair in Noord-Brabant raised one fledgling (estimated start of laying 28 April). White-tailed Eagle Haliaeetus albicilla: four pairs raised 7 chicks, i.e. one in the Oostvaardersplassen (2 chicks), one in Lauwersmeer (2 chick), one in Roggebotzand (1 chick) and one in Biesbosch (2 chicks). Except for Roggebotzand, all nests were in nature reserves. Marsh Harrier Circus aeruginosus: mean onset of laying was 3 May (range 15 April-29 June; most egg laying late April through early May, later pairs mostly repeat layings; Appendix 2, Table 2). Clutch size averaged 4.43 (SD=0.93, N=74, range 2-7; Appendix 3), mean number of fledglings/successful pair 3.21 (SD=1.10 N=92, range 1-5; Appendix 4). 57 complete broods were sexed: 98 males and 77 females. Over the years 1996-2013, a biased secondary sex ratio has been prevalent (Table 3: 53.1% males in 1374 nests with 4371 nestlings). Illegal nest destruction was recorded nine times. Food remains found on nests showed 50% birds, 49% mammals and 1% amphibians (N=213). Hen Harrier Circus cyaneus: the population continues to decline. A single nest was recorded on Vlieland (5 eggs resulted in three male fledglings), with two more in farmland in the the province of Groningen (failed); Texel numbers not yet known. Montagu’s Harrier Circus pygargus: in 2013, the Dutch population amounted to 36 pairs, raising 39 young, a significant decline compared to 2011 (63 pairs) and 2012 (49 pairs). Goshawk Accipiter gentilis: mean start of laying was 8 April (only 20% of 185 clutches started in March; range 21 March-28 April, Appendix 2), clutch size averaged 3.15 eggs (SD=0.75, N=157, range 1-4; Appendix 3), brood size (at fledging) 2.45 (Appendix 4, SD=0.82, N=308, range 1-5). Over the years, start of egg laying has declined (1984-2013 in Drenthe) or remained rather stable (1996-2013 in The Netherlands, including newly colonized areas in the west). Late starts were associated with low temperatures in March (Fig. 10). Secondary sex ratio in 178 nests was in favour of males (56.3%), typical for the Dutch population at large (mean for 1996-2013 55.1% male in 4516 nests with 12,022 nestlings). The local population in West-Drenthe (45 km2) showed stable numbers in 1990-96 (11-16 pairs), and a steady decline afterwards (all-time low in 2013 with 5 pairs), the decline being associated with an increase in the proportion of non-laying pairs, an increasing delay in laying date, and a decline in clutch size, brood size and fledglings per (successful) pair (Appendix 9). This trend has become typical for much of the breeding areas on sandy soils, formerly the core breeding range of Goshawks in The Netherlands, and is thought to be associated with a decline in avian biomass (excluding geese) in much of the eastern Netherlands. Among identified causes of nest failure in The Netherlands, illegal human activities were important (11 out of 29). Food remains found on and near nests showed a preponderance of pigeons (mostly Racing Pigeon Columba livia and Woodpigeon C. palumbus; 42.4% of 1303 prey remains; Appendix 5). Other important prey groups (in numbers) were: corvids including Jay Garrulus glandarius (17.9%), thrushes (9.1%) and Starling Sturnus vulgaris (5.4%). Sparrowhawk Accipiter nisus: mean onset of laying was 3 May (range 11 April-23 May, SD=7.51, N=112; Appendix 2); the proportion of pairs starting in April was 35%, indicating that spring conditions had been adverse. Clutch size averaged 4.59 (SD=0.75, N=95, Appendix 3), the number of fledglings per successful pair 3.86 (SD=1.15, N=144, Appendix 4). Among age-identified breeding birds, the proportion of first-years was rather high, i.e. 21% in males (N=43) and 25% in females (N=67), perhaps indicating high winter mortality. The secondary sex ratio among fledglings was in favour of males (55.2% males, 377 young, 106 nests). The long-term sex ratio was 51.2% male (1996=2013: 12,331 chicks on 3246 nests). Among nest failures, predation was most often recorded: 48x among 50 cases of natural losses. Buzzard Buteo buteo: average start of egg laying was calculated at 14 April (range 21 March-13 May, SD=7.94, N=375; Appendix 2), later than ever recorded since 1996 and indicative of prolonged adverse weather in spring in combination with a vole low. In the past winter, high mortality was recorded, also among adult breeding birds (normally, most birds found dead in winter are first-years). Mean clutch size was 2.14 (SD=0.59, N=276, with only 4 C/4), mean brood size of successful pairs was 1.61 (SD=0.61, N=643; Appendix 4). For the province of Drenthe, in 1990-2013 a steady decline in the number of broods with 4 eggs or 4 chicks was recorded. Although vole peaks were still visible thoughout this period, peaks became progressively lower in the course of time, indicating declining food suppy (mostly Cmmon Vole Microtus arvalis) perhaps in combination with density-dependent processes (population stable during this period, with indications of decline in later years). In 2013, in 30.5% of 1114 nests no eggs were laid (in 2012: 22.9%). Almost half of all recorded nest failures could be attributed to deliberate disturbance by man (34 out of 74 cases). The secondary sex ratio in 142 nests was male-biased: 110 males, 93 females (as in the longer run, i.e. 1996-2013: 53.7% for 6861 young on 3633 nests). Food choice was varied, with 48 bird species (41% of all prey items), 16 mammal species (59% of all prey items, Moles Talpa europaea, Rabbits, Hares and Common Voles most important), and some snakes (Vipera berus, Natrix natrix), Anguis fragilis, frogs, toads and fish (Appendix 6, N=904). Although Common Voles made up a small proportion of total diets (certainly underrecorded withn our method of diet analysis), numbers varied annually and were indicative of reproductive output in Buzzards (Fig. 13). Osprey Pandion haliaetus: nesting attempts were not recorded. Eurasian Kestrel Falco tinnunculus: onset of laying averaged 6 Mayl (range 7 April-3 July, N=262; Appendix 2), later than ever (see also Buzzard). Mean clutch size was 4.62 (SD=0.91, N=256; Appendix 3), mean number of fledglings/successful nest 3.88 (SD=1.24, N=346, Appendix 4). Many pairs refrained from egg-laying. In two large plots in the northern Netherlands, Kestrels showed a clear decline, despite the fact that providing nestboxes to some extent masked the decline (but didn’t stop it). Nest predation has become an important feature in these areas, at first mostly by Goshawks and Buzzards but increasingly also by Stone Martens Martes foina (and disturbances and nest take-overs by Egyptian Geese Alopochen aegyptiacus). Nest failures included human disturbance (4x) and natural causes (27x). Prey remains in nestboxes consisted mostly of birds (72% of 156 remains), the rest mainly being voles (Table 11). Hobby Falco subbuteo: mean start of laying was 11 June (range 29 May - 9 July, N=27; Appendix 2). Mean clutch size was 2.67 (SD=0.47, N=9; Appendix 3), mean number of fledglings/successful nest 2.42 (SD=0.70, N=50 Appendix 4). Sex ratio in 4 nests was 6 males and 4 females. Of 78 nests with known outcome, 55 were successful. Most pairs nested on old crow’s nests (61 of Corvus corone, out of 64 nests). Mean nest height excluding electricity pylons was 19.6 m (SD=7.9, N=45). In 2013, the proportion of pairs nesting on crow’s nests in electricity pylons was 31.9% (N=84 nests). Peregrine Falcon Falco peregrinus: lay date averaged 8 March (range 3 March - 9 April, with an outlier on 21 April), i.e. one month earlier than Goshawks (usually referred to as an early species). Clutches were C/2 (N=2), C/3 (N=4) and C/4 (N=9), brood size was on average 2.59 (SD=0.91, range 1-4, N=22). At four breeding sites in the SW-Netherlands, extensive prey lists were collected, mainly consisting of migratory birds, Racing Pigeons and Starlings. The Dutch population is still increasing, but more slowly than in the previous decade.
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