Trends en broedresultaten van roofvogels in Nederland in 2002

Slightly over 4000 nest cards of raptors were submitted in 2002 (Appendix 1), covering 11 species. The weather in 2002 was on average warm, sunny and wet, the summer index being 73.7 (a warm summer, based on temperatures in May-August) and a frost index of 9.4 (a mild winter, based on temperatures in November-March 2001/2002). Many prey species registered low numbers in 2002, among which all voles and mice (especially Microtus arvalis, Clethrionomys glareolus and Apodemus sylvaticus), rabbits and hares. Bird numbers were probably average. Social hymenoptera had an average year (code 3 on a scale of 1-5). The poor prey base was reflected in the much lower numbers of nestling raptors ringed (Table 1), i.e. 6895 in 2002 (not yet completed) compared to 9218 in 2001. European Honey-buzzard Pernis apivorus: the slowly declining trend of the last decade was reversed in 2002, with rather high numbers of adult birds cruising over woodland during summer. Apparently, many novice birds had entered the population, although few started egg laying. Mean onset of laying was 27 May (20 May-9 June, SD=5.11, N=26; Appendix 2). All egg-laying pairs visited during incubation produced a clutch of two eggs (Appendix 3), and the number of fledglings per successful pair averaged 1.73 (SD=0.45, N=30; Appendix 4). Eight nestlings on four nests were sexed (based on body mass, and after 35 days old, a check on these sex identifications based on DNA not yet received): 4 males and 4 females. Of 31 nests checked, 25 were built by Honey- buzzards, 4 by Common Buzzards Buteo buteo, 1 by Eurasian Sparrowhawk Accipiter nisus and 1 by Carrion Crow Corvus corone. Three failures could be attributed to (partial) predation, probably by Northern Goshawk (nestling stage) and Common Buzzard (egg stage). Eurasian Marsh Harrier Circus aeruginosus: mean onset of laying was 23 April (SD=10.9, N=122; Appendix 2), with some variation between regions (Table 3). Clutch size averaged 4.61 (SD=1.01, N=95; Appendix 3), mean number of fledglings/successful pair 3.24 (SD=1.16, N=138; Appendix 4). All surviving nestlings in 104 nests were sexed: 174 males and 151 females. Over the years, a male-biased sex ratio has been prevalent (Table 4: 655 nests with 2151 nestlings). Among prey remains found on nests, birds were particularly important (Table 5), partly caused by biased sampling but also indicative of poor vole numbers. Hen Harrier Circus cyaneus: population in steady decline, with only 4 pairs remaining in the Oostvaardersplassen and all other pairs residing on Wadden Sea islands, especially Texel and Terschelling, with few reproducing pairs left on Vlieland (2; one adult male killed by Northern Goshawk and replaced by immature), Ameland (4 nests, of which 3 successful and raising a total of 8 young) and Schiermonnikoog (handful). Nine pairs started egg laying on average on 30 April (SD=6.74; Appendix 2). Mean clutch size was 4.12 (including a C/1; Appendix 3), mean number of fledglings/successful pair 2.56 (Appendix 4). Bigamy was recorded on the Wadden Sea Islands of Texel and Ameland (once each). Montagu’s Harrier Circus pygargus: 36 pairs were recorded for The Netherlands, of which the majority nested in the province of Groningen (28). Average onset of laying was 27 May (SD=6.26, N=27, Appendix 2). Mean clutch size was 3.6 (SD=0.98, N=17; Appendix 3), mean number of fledglings/successful pair 2.25 (SD=0.70, N=20; Appendix 4). Voles, small passerines and hares are important prey groups. More information can be found on www.sovon.nl Northern Goshawk Accipiter gentilis: the latest breeding bird atlas, with fieldwork in 1998- 2000, recorded Goshawks in 53% of all 5 km squares; the number of pairs of 1800-2000 has been stable for some time. Some increase is still being recorded along the fringes of its distribution, as in Zeeland (two breeding pairs in 2002, being the first ever recorded for this province) and on the Wadden Sea Islands (10 on Texel, 2 on Vlieland, 2 on Schiermonnikoog, not yet on Arneland, no information for Terschelling). Regional variations in laying date (Table 6) may reflect variations in prey abundance, as well as the impact of temperature in March, Mean March temperature shows a clear positive correlation with laying date in Drenthe (Fig. 1), and this may partly explain the consistently earlier laying dates in the southern Netherlands (Noord-Brabant and Limburg; Table 6). Locally, Goshawk numbers have been declining since at least the early 1990s, as in western Drenthe (Table 7). However, no declines were noticed in clutch size and number of fledglings per (successful) pair. These study populations consist entirely of breeding birds in adult plumage, sometimes of considerable age. The decline is likely caused by declining prey stocks, but an increasing overall age of breeding birds may also affect reproductive performance. On average, Dutch Goshawks started egg laying on 1 April (range 16 March-29 April, SD=7.67, N=287; Appendix 2). Mean clutch size was 3.2 (SD=O.73, N=239; Appendix 3), mean number of fledglings/successful pair 2.59 (SD=O,84, N=377; Appendix 4). Among 44 identified nest failures, 19 were human caused. Take-overs by Egyptian Geese Alopochen aegyptiacus are recorded in increasing frequency. As in Drenthe (Table 7), most breeding birds elsewhere in The Netherlands were in adult plumage, i.e. 95.7% of 45 males and 96.3% of 163 females. Among surviving nestlings, sex ratio was highly male-biased in 2002: 392 males and 290 females on 263 nests. Since 1997, all nestlings were sex-identified on 2062 nests, resulting in a male-biased sex ratio of 55.2% (Table 8). Most important prey groups during the breeding season are pigeons (especially Racing Pigeon Columba livia and Woodpigeon C. palumbus), thrushes Turdus sp.. Starling Sturnus vulgaris and corvids Corvidae, but a wide range of other birds is being captured (Appendix 11). Sparrowhawk Accipiter nisus: in 1998-2000, 72% of all 5 km-squares in The Netherlands were occupied by breeding Sparrowhawks, including many city centres in the densely human-populated western half of the country. Mean onset of laying was 2 May (10 April-10 June, SD=8.9, N=264; Appendix 2), with distinct regional variations (Table 9, Appendix 7). Overall, 12% of 50 age- identified nesting males were in first-year plumage, among 129 females this was 19.4%. High turnover rates are known for some regions in The Netherlands (as evident from the proportion of breeders in their first year of life), but for most areas such information is lacking (Appendix 7). For the first time since 1996. the secondary sex ratio was male-biased (Table 10; 53.8% male, 923 nestlings on 234 nests). Common Buzzard Buteo buteo: the breeding bird atlas for 1998-2000 revealed a wide distribution, with 84% of all 5 km-squares being occupied by 8000-10,000 pairs. The breeding population consists almost entirely of birds in adult plumage, i.e. 100% of 134 males and 98.8% of 173 females. Many pairs refrained from egg laying in 2002, presumably caused by food shortage. Over the years, onset of laying correlated with vole index (Figure 2): laying dates advanced when voles were abundant. On average, egg laying pairs started later than usual (7 April, SD=7.82, N=742; Appendix 2), produced small clutches (mean 2.32, SD=0.60, N=474; Appendix 3) and few fledglings (mean 1.88/successful pair, SD=0.67, N=1037; Appendix 4). Other indications of food stress were visible in the profusion of fault bars in remiges and rectrices, the high incidence of starvation among nestlings and large within-nest age differences. Prey remains found on nests showed a high proportion of birds, with a scattering of amphibians, reptiles, fishes and mammals as well (Appendix 11). The relatively small numbers of voles and rabbits were telling evidence that their main prey species were in short supply. The secondary sex ratio was highly male-biased: 59.6% of 352 nestlings on 198 nests. Over the years 1996-2002, the male bias was on average 52.9% but with clear annual variations (3049 young on 1548 nests; Table 12). Osprey Pandion haliaetus. an adult pair (both ringed, the female even colour-ringed) started nest- building in mid-July 2002 in the Oostvaardersplassen, a large wetland in the central Netherlands. This was the first ever breeding attempt in The Netherlands, but no eggs were laid and the nest was demolished during a late October storm (cf. Bijlsma & de Roder 2002). Eurasian Kestrel Falco tinnunculus: many pairs did not lay eggs. In SE-Drenthe, for example, 30 of 78 nest boxes remained unoccupied. Onset of laying averaged 27 April, with a wide range from 23 March through 5 June (SD=13.1, N=465; Appendix 2). Mean clutch size was 5.06 (SD=0.86, N=435; Appendix 3), mean number of fledglings/successful nest 4.23 (SD=1.20, N=641). Local variations in these parameters may stem from variations in vole abundance (Table 13). Kestrels breeding in nest boxes had a higher nest success (88.7% of 589 nest boxes) than Kestrels using nests of corvids (77.0% of 13 nests). Eurasian Hobby Falco subbuteo: mean start of laying was 11 June (range 26 May-10 July, SD=9.27, N=40; Appendix 2). Mean clutch size was 2.30 (SD=0.51, N=21, Appendix 3), mean number of fledglings/successful nest 2.21 (SD=0.74, N=71; Appendix 4). Over 1997-2002, only 5.7% of 298 clutches were started in May. Clutch size showed a clear decline by laying date (data from 1997-2002 combined), especially after 11 June when clutches of four eggs are nor produced anymore (Table 15, Figure 3), Equally, fledgling production per pair declined significantly with laying date (Table 15, Fig. 4). Secondary sex ratio in 14 nests was almost equal: 16 males and 14 females (sexes identified by body mass and vocalisations). Nestlings were sexed in 100 nests in 1996-2002: 120 males and 123 females. Out of 82 nests, 74 were provided by Carrion Crows. Out of 86 nests, 19 were situated in pylons of high tension wires. Peregrine Falcon Falco peregrinus: in 2002, the Dutch population again increased slightly, to 10 pairs (all breeding in nest boxes). One pair apparently relaid after an initial failure, producing 3 fledglings (sexes not identified). Overall, 30 nestlings were raised to fledging, of which 14 males and 13 females were sex-identified (Table 17). Most nestlings were also colour-ringed. Mean onset of laying was 13 March, ranging from 26 February through 4 April. Mean clutch size was 3.8 (SD=0.6, N=7; Table 17), mean number of fledglings/successful pair 3.3 (SD=0.7, N=9; Table 17). Since 1990, 118 nestlings fledged, most of them being colour-ringed as well. A female nestling raised in The Netherlands in 2000 was found breeding NW of Warszawa in Poland in 2002; she successfully fledged 3 nestlings with her Polish mate.