Trends en broedresultaten van roofvogels in Nederland in 2016
De Takkeling , Volume 25 - Issue 1 p. 8- 60
A total of 3143 nest record cards of raptors were submitted in 2016 (Appendix 1, handed in up to and including 20 February 2017), covering 14 species. The preceding winter was very mild (frost index of 3.2 on a scale of 1-100). The summer was very warm (summer index 79.7). Indices of abundance of prey groups were collected in several non-random parts of the country. Voles (Microtus arvalis and Myodes glareolus) and mice (Apodemus sylvaticus) were again down in numbers after the steep peak of 2014, also in Friesland where high numbers had prevailed in 2014 and 2015. Rabbit Oryctolagus cuniculus numbers were low. Bird numbers in June-August were rather low, perhaps following high reproductive failure in the first part of the breeding season. Social wasps were present in low and moderate numbers (locally higher, though), high losses during the embryo stage of colonies but steep increases well into October and November of the surviving colonies of mostly Vespula vulgaris (although eventually often depredated by badgers and martens). Honey Buzzard Pernis apivorus: onset of laying averaged 26 May (range 23 May-7 June). Clutch size was 11x 2 eggs. Mean brood size was 1.5 (n=12, all broods with 1 chick started with 2 eggs). 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 (1 and 3 fledglings). 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 attempt to lay eggs. Marsh Harrier Circus aeruginosus: mean onset of laying was 25 April (range 7 April-24 May). Clutch size averaged 4.51 (n=51), mean number of fledglings/successful pair 3.14 (Appendix 3 and 4). 55 complete broods with 166 nestlings were sexed: 53.0% male. Illegal nest destruction was recorded nine times, nest predation eight times. Food remains found on nests showed a diverse diet (Appendix 5). Hen Harrier Circus cyaneus: the total Dutch population declined to 10 pairs, of which 4 in Groningen (mainland) and 6 on Wadden Sea Islands (3x Texel, 1x Vlieland, 2x Terschelling). A total of 2x 3 and 2x 4 nestlings fledged. Colour-ringing showed natal and breeding dispersal among Wadden Sea Islands. Montagu’s Harrier Circus pygargus: the Dutch population amounted to 44 pairs, raising 47 young by 25 successful pairs. Goshawk Accipiter gentilis: mean start of laying was 1 April (n=165, of which 52% started in March), clutch size averaged 3.44 eggs (of which 7x C/5 among 154 clutches), brood size (at ringing/fledging) 2.73 (only 1x 5 chicks, among 236 nests). Onset of laying was slightly earlier in the south of the country, but the discrepancy between clutch and brood size was also larger there. Sexratio on 147 nests was in favour of males (217 males, 186 females), in accordance with a long-term male preponderance (1996-2016: 4939 nests, 13,153 young, 55.2% male). Nest failures were human disturbance (1x), and natural (11x, of which once a take-over by Alopochen aegyptiaca). The prey list (n=1110) contained almost exclusively birds (99.6%), of which pigeons were numerically by far the most important (46.8% of all prey). Sparrowhawk Accipiter nisus: mean onset of laying was 29 April (range 14 April-21 May, n=88), with 63% of pairs starting in April. Clutch size averaged 4.86 (n=100), the number of fledglings per successful pair 4.03 (n=104). The secondary sex ratio among fledglings was almost fifty-fifty (129 males, 126 females, 65 nests). A decline is in full swing, with in 2016 only some 30% of the nest cards submitted in comparison to the late 1990s (i.e. cards from which sex ratio of nestlings was recorded). Among nest failures, predation was important: 15x, compared to 1x desertion, 3x adverse weather and 4x human disturbance. Buzzard Buteo buteo: average start of egg laying was calculated at 6 April (range 12 March-7 May, n=345; 24% of all pairs started laying in March). Mean clutch size was 2.44 (n=315, with 13 C/4), mean brood size of successful pairs was 1.87 (n=576, with only 7 B/4). Secondary sex ratio in 107 nests with 197 chicks was 52.8% males. Of 1017 nests closely followed, 26.4% failed to fledge chicks. Known causes of nest failure included deliberate nest disturbance by humans (26x) and natural causes (19x). Diet was varied, with 39 bird species (23.3% of 842 prey items), 16 mammal species (73.2% of all prey items, lagomorphs and moles being important in terms of biomass), and some snakes, frogs, toads and fish. Osprey Pandion haliaetus: first successful nest in The Netherlands (De Biesbosch) fledged a single chick; another Biesbosch-pair built nests but did not lay eggs. Kestrel Falco tinnunculus: onset of laying averaged 1 May (range 21 March-9 June, n=458). Mean clutch size was 4.94 n=496), mean number of fledglings/successful nest 4.13 (n=623). The effect of the vole peak in 2014 (extending into 2015 in Friesland) had disappeared (as recorded via start of laying and reproductive performance). Nestfailures included human disturbance (5x) and natural causes (20x, of which 1x takeover by Egyptian Geese). Voles were important among 993 prey found in nestboxes and pellets (68%). A wide variety of birds (23 species) was also recorded as prey. Hobby Falco subbuteo: mean start of laying was 11 June (range 30 May-6 June, n=18). Mean clutch size was 1x 2 and 5x 3, mean number of fledglings/successful nest 8x 1, 28x 2, 18x 3 and 1x 4. Sex ratio in 3 nests was 2 males and 5 females. Of 80 nests with known outcome, 65 were successful. Most pairs nested on old crow’s nests (39x Corvus corone, 1x Buteo buteo, 2x Pica pica, 1x Corvus ftrugilegus, and 2x in basket). The proportion of pairs nesting on crow’s nests in electricity pylons was 37% (n=73 nests). Peregrine Falcon Falco peregrinus: lay date averaged 19 March (range 3 March-20 April, n=24). Clutch size was 1x 2, 5x 3, 14x 3 and 2x 5 eggs, brood size averaged 2.56 (n=34). Secondary sex ratio on 20 nests was 31 males and 26 females. Extensive prey lists from breeding sites in Zeeland and Limburg showed a preponderance of racing/feral pigeons (Appendix 8). Regional variation in diets: exemplified by data for breeding Buzzards, collected throughout The Netherlands in 1997-2015. Out of 23,000 prey items collected on/ near nests, 29 mammal species were identified with 12,608 items, 109 bird species with 9522 items, and 24 reptile, amphibian and fish species with 870 items. Regional variation in diets was huge, associated with soil characteristics. The reproductive performance of Buzzards is largely dictated by Common Voles, hence the regional and annual variations in reproductive output. Trends in The Netherlands: raptors are in a much better state than 100 years ago when protection was non-existent. However, after steep increases since the 1970s, partly following recoveries from depleted populations after the use of persistent pesticides was successively banned, several species are now in decline, particularly those breeding on sandy soils in the eastern part of the country. The Sparrowhawk is a case in point. Since the late 1990s, the number of submitted nest cards halved, and the number of nestcards from which the sex ratio among nestling could be deduced dropped to some 30%. As the breeding performance, expressed as clutch size and number of fledglings/successful pair, did not decline, it is thought that numerical declines were caused by predation (which has increased substantially). Other species, like Goshawk and Buzzard, also showed declines, albeit not in the same range as Sparrowhawks, notwithstanding considerable range expansions across the country. Declines in these species are likely associated with declines in prey abundance.
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