Trends en broedresultaten van roofvogels in Nederland in 2011

A total of 3102 nest record cards of raptors were submitted in 2011 (Appendix 1, handed in up to and including 15 January 2012), covering 12 species. The preceding winter was normal (frost index of 23.6 on a scale of 1-100). The summer was normal (summer index 54.1), with high spring temperatures but much wetter and cooler from 5 June onwards). Vole ( Microtus arvalis and Myodes glareolus) and mice (Apodemus sylvaticus) abundance was rather low throughout the country, with small local variations. Rodent numbers only started to improve in autumn. Indices of Rabbit Oryctolagus cuniculus and Hare Lepus europaeus were again low, and bird numbers probably more or less stable. Social wasps were present in medium numbers with colonial activities peaking in early July and late July and early August (German and Common Wasp Vespula germanica and V. vulgaris, Saxon Wasp Dolichovespula saxonica). Honey-buzzard Pernis apivorus: onset of laying averaged 24 May (range 15 May-9 June), with a single peak in late May. Clutch size was 1x 1, 18x 2 egg and 1x 3 egg(s). Mean brood size was 13x 1 and 24x 2 chick(s). Of 57 nests recorded, eggs were laid in 51 nests, of which 13 failed (6x egg stage, 7x chick stage). Most causes of failure involved predation. The high proportion of single-chick nests is proof of problems in the latter stage of the breeding cycle, associated with adverse weather, perhaps in parallel with declining wasp numbers. Black Kite Milvus migrans: for the third year in succession a nesting attempt took place in the southern Netherlands (Limburg) where a single chick was successfully raised (out of two chicks). Another pair successfully raised two chicks elsewhere in the southern Netherlands. White-tailed Eagle Haliaeetus albicilla: the pair in the Oostvaardersplassen raised two chicks, the sixth successive breeding attempt here. The pair in Lauwersmeer raised a single chick, the first successful attempt here after last year’s failure. The third pair, in the lake district of the central Netherlands, failed to lay. In the winter of 2011/12 a fourth pair started nest-building in the Biesbosch (southern Netherlands). Marsh Harrier Circus aeruginosus: mean onset of laying was 28 April (range 10 April-20 June, the latter three weeks later than the last but one nest; Appendix 2, Table 3). Clutch size averaged 4.4 (SD=0.8, N=47; Appendix 3), mean number of fledglings/ successful pair 3.3 (SD=1.0 N=78, Appendix 4). All surviving nestlings in 52 nests were sexed: 78 males and 86 females. Over the years 1996-2011, a biased secondary sex ratio has been prevalent (Table 4: 52.8% males in 1243 nests with 3992 nestlings). Illegal nest destruction occurred in the province of Friesland. Food remains found on nests showed a diverse diet, with voles, rabbits and hares, and a variety of (fledglings of) birds. Hen Harrier Circus cyaneus: the population is in steady decline. The former haunts on the Wadden Sea Islands are nearing extinction, with only 8 pairs left on Texel, 1 on Vlieland, 3 on Terschelling, 0 on Ameland and 2 on Schiermonnikoog. Five pairs settled in farmland in the provinve of Groningen, of which two were successful (6 chicks raised, with the help of nest protection; nests located in luzerne and cereals). Montagu’s Harrier Circus pygargus: in 2011, the Dutch population amounted to 63 pairs, distributed across the provinces of Groningen (49), Flevoland (9), Friesland (3), Drenthe (1) and Overijssel (1, the first one since at least 70 years). Some 90 chicks fledged. Goshawk Accipiter gentilis: mean start of laying was 3 April (44% of 217 clutches started in March; range 16 March-16 May, Appendix 2), clutch size averaged 3.3 eggs (SD=0.8, N=207, range 1-5, including 3 C/5; Appendix 3), brood size (at fledging) 2.6 (Appendix 4, SD=0.9, N=322, range 1-4). Secondary sex ratio on 212 nests was in favour of males (55.0%), which is in line with the mean for 1996-2011 (55.2% male in 4140 nests with 11,067 nestlings). Among identified causes of nest failure, 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; 39.0% of 1174 prey remains; Appendix 5). Among ageidentified Racing Pigeons, first-year birds were most often recorded (64% of 50 rings found on nests and in pellets). Other important prey groups (in numbers) were: corvids including Jay Garrulus glandarius (19.6%), thrushes (10.1%), Starling Sturnus vulgaris (4.3%), Rabbit Oryctolagus cuniculus (4.3%) and raptors and owls (4.2%). Sparrowhawk Accipiter nisus: mean onset of laying was 30 April (range 15 April-23 May, N=76; Appendix 2); the proportion of pairs starting in April was 62%. Clutch size averaged 5.0 (SD=0.8, N=62, Appendix 3), the number of fledglings per successful pair 4.0 (SD=1.1, N=111). Of two nests with 7 eggs, none raised 7 chicks (Appendix 4). The secondary sex ratio was in favour of males (140 males, 132 females, 64 nests). The long-term sex ratio was 51.1% male (1996=2011: 11,581 chicks on 3041 nests). Among nest failures, predation was most often recorded (in one study site in Limburg, for example, only one nest out of 14 succeeded in fledging its chicks). Buzzard Buteo buteo: average start of egg laying was calculated at 6 April (range 19 March-4 May, N=533; Appendix 2). Mean clutch size was 2.5 (SD=0.6, N=404, with twelve C/4), mean brood size of successful pairs was 2.0 (SD=0.7, N=894; Appendix 4). The secondary sex ratio in 139 nests was slightly male-biased: 193 males, 186 females. Sex ratio (% males) declined with increasing vole numbers (based on 1996- 2011). Food choice was varied, with 53 bird species (43.5% of all prey items), 19 mammal species (53.2% of all prey items. Moles Talpa europaea, Rabbits, Hares and Common Voles most important), and a scattering of snakes, lizards, frogs, toads and fish (Appendix 6, N=1317). Out of 115 identified causes of failure, 59 could be attributed to deliberate human intervention (mostly in the province of Friesland). Osprey Pandion haliaetus: nesting attempts were not recorded. Eurasian Kestrel Falco tinnunculus: onset of laying averaged 20 April (range 23 March-20 June, N=369; Appendix 2). Mean clutch size was 5.2 (N=356; Appendix 3), mean number of fledglings/successful nest 4.5 (N=485, Appendix 4). Regional variations in reproductive output were small, reflecting medium vole densities across The Netherlands. Out of 483 registered breeding sites, 467 were in nest boxes, the rest mainly in nests of Carrion Crows. Causes of failure were mostly natural, i.e. 28 out of 40 (especially predation). Of prey remains found in nestboxes, birds consisting of 20 species covered 35% of 171 prey species, the rest were mainly small mammals. Hobby Falco subbuteo: Mean start of laying was 9 June (range 4-19 June, N=23; Appendix 2). Mean clutch size was 2.67 (N=7; Appendix 3), mean number of fledglings/successful nest 2.4 (N=52 Appendix 4). Secondary sex ratio in 1996-2011 was 46.5% males (sexes identified by body mass and vocalisations, 419 chicks on 175 nests). In 2011, the proportion of pairs nesting on crow’s nests in electricity pylons was 32.9% (N=73 nests). Peregrine Falcon Falco peregrinus: the number of breeding pairs was estimated at 100-125, an increase from the 90 pairs breeding in 2010. Lay date averaged 19 March (range 3 March-18 April). Clutches were C/3 (N=2) and C/4 (N=4), brood size was on average 2.2 (range 1-4, N=22). Of 30 nestlings sexed in 13 nests, nine were male. For the first time since the 1970s, a tree-nesting Peregrine was detected (raising one chick, with the help of raptorphiles).