Trends en broedresultaten van roofvogels in Nederland in 2010

A total of 2708 nest cards of raptors were submitted in 2010 (Appendix 1, up to and including 31 December 2010), covering 12 species. The preceding winter was normal (frost index of 26.6 on a scale of 1-100), but with prolonged snow cover and often below-zero temperatures until mid-March. The summer was warm (summer index 75.6, i.e. within 0.3 of very warm). Vole ( Microtus arvalis and Clethrionomys glareolus) and mice ( Apodemus sylvaticus) abundance was high in the first part of winter, but collapsed to very low values in the second half. No improvement in rodent numbers was recorded throughout the summer and autumn. Also, indices of Rabbit Oryctolagus cuniculus and Hare Lepus europaeus remained low, bird numbers in general were depressed (especially of residents) and social wasps were present in small numbers with colonial activities peaking in mid-July and largely finished by mid-/late August (German and Common Wasp Vespula germanica and V. vulgaris, Saxon Wasp Dolichovespula saxonica). In contrast to previous years, lows in food supply (voles, birds. Rabbits, wasps) were consistent throughout the country. Honey-buzzard Pernis apivorus: onset of laying averaged 28 May (range 19 May-8 June), with a single peak in 26-30 May. Over the years, onset of laying was positively correlated with arrival date and mean May temperature. Clutch size was 1x 1 and 16x 2 egg(s). Mean brood size was 14x 1 and 16x 2 chick(s), indicating widespread loss of one chick (invariably the second hatched). This result attests to the low wasp availability throughout summer, but few non-wasp prey remains were recorded on nests (non-invertebrate prey items are usually completely devoured in poor wasp years, in stark contrast to good wasp years). Black Kite Milvus migrans: for the second year in succession a nesting attempt took place in the southern Netherlands (Limburg) where a single chick was successfully raised. The female was presumed to be the same as in 2009, but the male had been replaced. White-tailed Eagle Haliaeetus albicilla: the pair in the Oostvaardersplassen raised a single chick (a female). This is the fifth consecutive breeding attempt here. The first (and only?) egg was laid on 11 March. Two other nesting attempts were recorded, i.e. in Lauwersmeer (failed in egg stage) and Zwarte Meer (failed in egg stage). This was the first time that more than one pair attempted to nest in The Netherlands. Marsh Harrier Circus aeruginosus: mean onset of laying was 26 April (range 9 April-10 June, the latter a repeat laying; Appendix 2, Table 3). Clutch size averaged 4.59 (SD=0.96, N=66; Appendix 3), mean number of fledglings/successful pair 3.36 (SD=1.02 N=89, Appendix 4). All surviving nestlings in 69 nests were sexed: 117 males and 110 females. Over the years 1996-2010, a biased secondary sex ratio has been prevalent (Table 4: 53.0% males in 1191 nests with 3828 nestlings). Nesting in farmland (often in cereals) has somewhat increased in the southestem Netherlands. Illegal nest destruction occurred in the province of Friesland. Food remains on nests consisted of birds (19 species, N=65), small mammals (5 species, mostly Common Voles, N=57) and fish (N=3) Hen Harrier Circus cyaneus: insufficient information was collected. The population is in steep decline. A novel phenomenon was recorded in the NE Netherlands, where three pairs settled in cereals; two of these pairs were successful (2 and 4 chicks raised). These birds may have been attracted by set aside measures. One of the adults (female) had been ringed as a nestling in nearby Germany in 2007. Montagu’s Harrier Circus pygargus: see www.grauwekiekendief.nl Goshawk Accipiter gentilis: mean start of laying was rather late in 2010 (4 April, 38% of 208 clutches started in March; range 17 March-2 May, Appendix 2), clutch size averaged 3.31 eggs (SD=0.84, N=170, range 1-5, including 4 C/5; Appendix 3), brood size (at fledging) 2.54 (Appendix 4, SD=0.90, N=288, range 1-5, of which 1 B/5). Secondary sex ratio on 190 nests was in favour of males (54.9%), and in line with the long-term mean for 1996-2010 (55.2% male in 3928 nests with 10,516 nestlings). Among identified causes of nest failure, illegal human activities were important (15 out of 27), with natural causes being desertion (4x), egg predation (3x), and chick predation (1x). Food remains found on and near nests showed a preponderance of pigeons (mostly Racing Pigeon Columba livia and Woodpigeon C. palumbus; 37.8% of 1198 prey remains; Appendix 5). Among age-identified Racing Pigeons, first-year birds were most often recorded (66.7% of 42 rings found on nests and in pellets). Other important prey groups (in numbers) were: corvids including Jay Garrulus glandarius (21.8%), thrushes (11.1%) and Starling Sturnus vulgaris (4.9%). Sparrowhawk Accipiter nisus: mean onset of laying was 30 April (range 14 April-27 May, N=102; Appendix 2); the proportion of pairs starting in April was 57%. Clutch size averaged 4.82 (N=86, Appendix 3), the number of fledglings/successful pair 3.86 (N=124). Only one nest held 7 eggs, but none of the nests fledged 7 chicks (Appendix 4). The secondary sex ratio was in favour of males (191 males, 153 females, 143 nests). The long-term sex ratio was 51.1% male (1996=2010: 11,309 chicks on 2977 nests). Among breeding birds in 2010, the proportion of first-years was high in females, i.e. 32% (N=25). Buzzard Buteo buteo: average start of egg laying was calculated at 6 April (range 17 March-12 May, N=470; Appendix 2). Mean clutch size was 2.34 (N=318, with only 5 C/4 and 2 C/5), mean brood size of successful pairs was 1.82 (N=729; Appendix 4). Clutch and brood size showed a strong seasonal decline. The secondary sex ratio in 139 nests was male-biased: 60.2% of 211 young. Food choice was varied, with 55 bird species (47.2% of all prey items), 18 mammal species (55.2% of all prey items. Moles, Rabbits, Hares and Common Voles being most important), and a scattering of snakes, lizards, frogs, toads and fish (Appendix 6, N=1010). Out of 94 identified causes of failure, 39 could be attributed to deliberate human intervention (mostly in the province of Friesland). Natural causes of failure included desertion (7x), egg predation (21x), chick predation (15x), adverse weather (5x) and take-overs and disturbances by Egyptian Geese Alopochen aegyptiacus (7x). Osprey Pandion haliaetus: nesting attempts were not recorded. Eurasian Kestrel Falco tinnunculus: onset of laying averaged 24 April (range 26 March-10 June, N=3211; Appendix 2). Mean clutch size was 5.11 (N=282; Appendix 3), mean number of fledglings/successful nest 4.52 (N=412, Appendix 4). Regional variations in reproductive output were rather small, indicating an overall low in voles numbers. Clutch and brood size showed a clear seasonal decline. Out of 460 registered breeding sites, 445 were in nest boxes, and only 10 on nests of other birds (5 others in buildings). Causes of failure were mostly natural, i.e. desertion (7x), predation (5x eggs, 3x chicks, 1x parent) and take-overs by Egyptian Goose (3x). Of prey remains found in nestboxes, small mammals predominated (5 species, N=97, mostly Microtus arvalis), but birds were also prolificly represented (13 species, N=34). Hobby Falco subbuteo: Mean start of laying was 10 June (range 3-21 June, N=25; Appendix 2). Mean clutch size was 2.67 (N=9; Appendix 3), mean number of fledglings/successful nest 2.26 (N=39; Appendix 4). Secondary sex ratio in 1996-2010 was 46.7% males (sexes identified by body mass and vocalisations, 411 chicks on 172 nests). The proportion of pairs nesting on crow’s nests in electricity pylons was 27.9% (N=60 nests). Over 1999-2010, nest success of pylon breeding Hobbies has been similar (76.2% in N=239) to that of tree nesting Hobbies (78.9%, N=790). Peregrine Falcon Falco peregrinus: details will be published by the Peregrine Working Group. Probably more than 90 pairs bred in 2010.