Trends en broedresultaten van roofvogels in Nederland in 2006

A total of 3364 nest cards of raptors were submitted in 2006 (Appendix 1), covering 12 species (Peregrine not yet available). The relatively low number of nest cards reflects a low in the vole cycle, which particularly affected Buzzard and Kestrel. The preceding winter was mild (frost index of 14.0), spring on the other hand showing cold spells alternated with warmer episodes. Similarly, mid-May and early June had brief cold spells but July was exceedingly warm; August was extremely wet. Overall, the summer was hotter than ever before, as shown by the summer index of 107.9 (theoretically, this index cannot exceed 100). A year with extremes, in short. Food supply varied from very poor (voles, mice) to poor (rabbit, hare), rather poor (birds) and abundant (social wasps, with German wasp Vespula germanica peaking late July, and common wasp V. vulgaris in August). European Honey-buzzard Pernis apivorus: onset of laying averaged 27 May, with a large range of 16 May through 16 June, possibly the result of a cold spell in early June. Only clutches with 2 eggs were recorded (N=16), and mean brood size was 1.8 (SD=O.4, N=30). Chick condition was generally very good, as expected from the good wasp numbers (index 3 on a scale of 1-5). Food items on nests consisted almost entirely of wasp combs (N=231), together with 2 frogs and 1 Turdus merula. Out of 39 nests, 22 were built by Honey-buzzards, the other nests by Common Buzzard (6), Northern Goshawk (3) and Sparrowhawk (1). Mean nest height was 14.1 m (SD=3.69, N=42). Black Kite Milvus migrans: a failed breeding attempt was recorded in the southern Netherlands (nest incubated, but deserted between 5 and 7 June; Don 2006). Another possible attempt was based on a solitary bird carrying twigs, with regular observations elsewhere in the same region of two or three birds; a nest, however, was not unequivically found (Wester 2006). White-tailed Eagle Haliaeetus albicilla: a successful breeding attempt was recorded in the Oostvaardersplassen, the first ever in the Netherlands. The female was colourringed as a nestling in 2003 in northern Germany; the adult male was not banded. A single chick fledged on 19 July, the egg possibly being laid on 26 March on a selfmade nest in a willow (7.4 m high). Prey mainly consisted of geese, ducks, coots and fishes (de Roder & Bijlsma 2006). Marsh Harrier Circus aeruginosus: mean onset of laying was 24 April (range 7 April-27 May; Appendix 2, Table 4). Clutch size averaged 4.72 (SD=1.03, N=82; Appendix 3), mean number of fledglings/successful pair 2.95 (SD=0.92, N= 117; Appendix 4). All surviving nestlings in 92 nests were sexed: 155 males and 109 females. Over the years, a biased sex ratio has been prevalent (Table 5; 52.8% males in 1013 nests with 2942 nestlings). Nest destruction is common practice in the province of Friesland, where annually at least 10% of the nests is destroyed. Hen Harrier Circus cyaneus: the decline of the Dutch population has continued, but the overall picture is not yet available. The only breeding site on the mainland, the Oostvaardersplassen, held two pairs that failed to raise chicks. The species has now almost disappeared from the island of Ameland (just a single nest successful with 3 chicks, out of 4 breeding attempts). On the island of Schiermonnikoog, three nests fledged 2x 4 and 1x 5 chicks. Numbers on the island of Terschelling have dramatically dropped, and very few pairs raised fledglings. The island of Texel is the only site where Hen Harriers are still doing quite well, although in 2006 six out of 18 females were first-years (3 successful, raising 1, 2 and 3 young respectively); of 12 adult females, 8 raised 28 chicks. Overall clutch size in The Netherlands averaged 4.41 eggs (range 3-5; Appendix 3), brood size 2.80 (range 2-3; Appendix 4). Mean onset of laying was 5 mei (range 30 April-8 May, N=6; Appendix 2). Montagu’s Harrier Circus pygargus: 39 pairs recorded for The Netherlands, mainly in Groningen (28), and smaller numbers in Flevoland (5), Lauwersmeer (3) and Drenthe (1). Mean onset of laying varied from 11 May (Lauwersmeer) to 21 May (Flevoland) and 23 May (Groningen). Mean clutch size was 3.5, mean number of fledglings/successful pair 2.4. All together, 50 nestlings fledged. Several adult males were radio-tagged to study hunting behaviour and habitat use. Six adults were fitted with a satellite transmitter, and were tracked into Africa (Senegal, Mauritania, Mali). See www.grauwekiekendief.nl for details. Goshawk Accipiter gentilis: the colonisation of the Wadden Sea Islands has now stabilised (Table 5), and only in the province of Zeeland (SW-Netherlands) some increase is still noted. In the eastern Netherlands, formerly the mainstay of Goshawks, numbers have halved during the past decade. Onset of laying was retarded in comparison with earlier years, with a mean date of 4 April (range 21 March-1 May, SD=7.43, N=236; 91% started between 22 March and 15 April; Appendix 3), Regional variations in laying date, with on average an earlier start in the southern and western Netherlands, are thought to reflect higher prey abundance in those regions (Table 6). In the southern Netherlands (Noord-Brabant + Limburg), for example, pigeons constituted 48% of the summer diet, compared with 30% in Gelderland and Drenthe (central and northern Netherlands, where Goshawks are known to have substantially declined in numbers) (Appendix 5). Mean clutch size was 3.33 (SD-0.77, N=207; Appendix 3), mean number of fledglings per successful pair 2,59 (SD=0.88, N=304; Appendix 4), Only one clutch with 5 eggs was recorded (in 2005: 15). Few breeding birds were in first-year plumage, i.e. 4.3% of 46 males and 4.4% of 114 females. Among surviving nestlings, sex ratio was male-biased in 2006; 333 males and 242 females on 223 nests. Since 1996, all nestlings were sex-identified on 3028 nests, resulting in a sex ratio of 55.3.0% (Table 8). Identified nest failures were mainly human-caused (48 out of 68). Sparrowhawk Accipiter nisus: mean onset of laying was 3 May (range 18 April-4 June, N=169; Appendix 2), with small regional variations (Table 9). Mean clutch size was 4.67 (N=139, Appendix 3), mean number of fledglings/successful pair 3.79 (N=204, Appendix 4). The secondary sex ratio was in favour of males (Table 9: 309 males, 218 females, 150 nests). Common Buzzard Buteo buteo: average start of egg laying was very late, i.e. on 10 April (range 23 March-17 May, N=544; Appendix 2). Mean clutch size was 2.33 (N=433, with only a single C/4), mean brood size of successful pairs was 1.69 (N=834; Appendix 4). The late start and small clutch and brood sizes are consistent with a low in vole numbers (as found in vole transects). Regional variations in onset of laying, clutch size and brood size were probably linked with vole abundance throughout the season (Table 10). Food choice was highly varied, with 49 bird species, 15 mammal species, 7 species of reptiles and amphibians, and some fishes (Appendix 6, N=917). Common voles Microtus arvalis are probably underrecorded (15.8% in number), as only in Friesland and Drenthe are nests being visited regularly during the early chick stage (see higher frequency of Common Voles here; Appendix 6). The secondary sex ratio was male-biased: 188 males and 139 females on 209 nests (Table 11). The proportion of males increased with increasing brood size: 54.5% in broods of 1 chick (N=112 young), 58.6% in broods with 2 chicks (N=162) and 61.4% in broods with 3 chicks (N=57). Out of 181 identified causes of failure, 131 could be attributed to deliberate human action. Especially in the province of Friesland, hunters and protectionists of meadowbirds collectively destroy up to 10% of all Buzzard nests each year. In 2006, two nest were recorded on electricity pylons (2 in 2005, 4 in 2003), and two nests were found on the ground in grassland (both failed). Osprey Pandion haliaetus: nesting attempts in the Oostvaardersplassen were not recorded (Frank de Roder), nor elsewhere. Eurasian Kestrel Falco tinnunculus: onset of laying averaged 3 May (range 31 March-13 June, N=375; Appendix 2). Mean clutch size was 4.79 (N=356; Appendix 3), mean number of fledglings/successful nest 4.06 (N=472, Appendix 4). The northem provinces fared slightly better that those in the south (Table 12). Out of 618 registered breeding sites, 604 were nest boxes. Nine pairs nested in old crow’s nests, of which three in electricity pylons. Causes of failure were mostly natural, i.e. desertion and predation. Eurasian Hobby Falco subbuteo: most nests were located in Friesland and Noord- Brabant, very few in extensive woodland in the eastern Netherlands (where the species used to be common). Present numbers appear to be stable at a much lower density than in the 1980s and 1990s. The proportion of pairs nesting on crow’s nests in electricity pylons has steeply increased between 1998 and 2006, with presently up to 30% breeding on pylons (Fig. 3). Mean start of laying was 12 June (range 1 June-6 July, N=39; Appendix 2). Mean clutch size was 2.88 (N=27; Appendix 3), mean number of fledglings/successful nest 2.32 (N=65; Appendix 4). Secondary sex ratio in 4 nests was in favour of females: 3 males and 7 females (sexes identified by body mass and vocalisations) (Table 14). Peregrine Falcon Falco peregrinus: the number of nesting pairs increased to 32. Details will be published by the Dutch Peregrine Group. For the first time since the early 1990s a pair successfully bred on the ground (R. Beijersbergen, this Takkeling).