Trends en broedresultaten van roofvogels in Nederland in 2005

Slightly more than 4000 nest cards of raptors were submitted in 2005 (Appendix 1), covering 10 species. Numbers of chicks ringed are shown in Table 1; the lower numbers compared with 2004 are caused by incomplete data (many ringers did not yet submit their records to the Ringing Station). 2005 had a summer index of 56.5 (i.e. rather warm, based on temperatures in May-August), but showed extended periods of adverse weather (wet April, below-average temperatures mid-May, cold early June, lots of rainfall in July and early August). The frost index was 12.4 (rather mild winter, based on temperatures in November-March 2004/2005). Many prey species registered above-average figures in early 2005, especially voles and mice. However, in several regions numbers crashed in the course of the summer. Rabbits showed some recovery from outbreaks of Rabbit Viral Haemorrhagic Disease, but many local populations are wiped out and numbers are still low except in local hotspots (mainly in the south and in built-up areas) Similarly, hares are in decline and have become scarce in many parts of the country. Social hymenoptera had a poor year (index 2 on scale of 1 -5), despite a promising start (many prospecting queens in May). Numbers and trends: apart from Common Buzzard (increase/stable) and Peregrine most raptor species are presently in decline in The Netherlands, especially so since the last decade and after steep increases in the 1970s and 1980s (Table 2). Trends differ according to region and species. In general, raptors breeding in the eastern Netherlands (formerly the mainstay of tree-nesting raptors) are in decline, those in the western Netherlands on the up (formerly largely devoid of raptors except Eurasian Kestrels) (Table 3). The discrepancy in trends, even within species, is mainly caused by regional variations in prey abundance (declines in the east), predation risks (highest in the east) and behavioural adaptations (breeding in open farmland and cities, a recent trend). European Honey-buzzard Pernis apivorus: breeding in the dunes of the western Netherlands is now fairly well established (3 nesting pairs in 2005). Overall in The Netherlands, onset of laying averaged 25 May (range 20 May-9 June, SD=4,84, N=21), in line with the generally earlier start in the past decade. Many broods showed food shortage, either as starvation of chick B (Photo 2) or as poor nestling conditions (low weights, many faultbars). Out of 259 prey remains collected on nests (1 wasp comb = 1 prey remain), 21 were vertebrate (mainly nestling birds and amphibians) and 238 invertebrate (mainly combs of Vespula germanica; Table 5). Out of 19 nests, 15 were built by Honey-buzzards, the other nests by Common Buzzard (2), Northern Goshawk (1) and Carrion Crow (1). Mean nest height was 13.6 m (SD=5.56, N=23). Eurasian Marsh Harrier Circus aeruginosus: mean onset of laying was 22 April (range 24 March-21 April; Appendix 3, Table 6). Clutch size averaged 4.60 (SD=1.01, N=70; Appendix 4), mean number of fledglings/successful pair 3.21 (SD=1.08, N=100; Appendix 5). All surviving nestlings in 50 nests were sexed: 74 males and 82 females. Over the years, a biased sex ratio has been prevalent (Table 7: 52.2% males in 821 nests with 2678 nestlings). The species shows a country-wide decline, also in the mainstay in the western Netherlands and on the Wadden Sea Islands (Fig. 1: trend for Ameland 1955-2005). In several breeding areas in open farmland, Common Buzzards have steadily increased up to the point that they are now omnipresent. Consequently, robbing prey-carrying male Marsh Harriers has become widespread, possibly impacting its breeding performance and distribution. Also, the ongoing industrialisation of farmland, widespread occurrence of Red Fox Vulpes vulpes in formerly fox-free regions, urbanisation and recreational activities, input of large herbivores in nature reserves and human persecution are bad news for Marsh Harriers. Hen Harrier Circus cyaneus: the decline in the Dutch population has continued, with only some 50 pairs left in 2005 (almost exclusively on the Wadden Sea Islands, Table 8). Clutch size averaged eggs (range 2-6; Appendix 4) and an average brood size of (range 1-5; Appendix 5). 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. Two adult females were fitted with a satellite transmitter, and were tracked into Africa (Morocco and Niger, respectively). See Trierweiler et al., this Takkeling, and www.grauwekiekendief.nl. Northern Goshawk Accipiter gentilis: the trend of the past decade (declines in eastern and central Netherlands, expansion and stabilisation in the northern and western Netherlands) is continuing. The colonisation of the Wadden Sea Islands has now almost stabilised (Table 9), and the only region still largely free of Goshawks is Zeeland (SW-Netherlands). Regional variations in laying date are thought to reflect variations in prey abundance. In Het Gooi, for example, almost all Goshawks started breeding in March (in 2005, a pair even started laying on 9 March, only a few days after heavy snowfall and -18°C) (Table 11). On the other hand, food scarcity in Drenthe resulted in a steady population decline (Table 10), and a decline in clutch size (Fig. 3). Long-term variations in onset of laying, clutch size and brood size are largely synchronous between regions (Figs. 2, 3, 4). Pigeons, thrushes, corvids and starling are most frequently preyed upon, locally augmented with ducks, waders, woodpeckers or rabbits. All in all, 55 species of birds and 5 species of mammals (only 55, out of a total of 1247 prey items) were recorded as prey (Appendix 10). Onset of laying averaged 2 April (range 9 March-18 April. N=269; Appendix 3). Mean clutch size was 3.51 (SD=0.78, N=212; Appendix 4), mean number of fledglings per successful pair 2.80 (SD=0.87, N=348; Appendix 5). Fifteen clutches with 5 eggs were recorded, evenly spread over the country. Three of these C/5s resulted in 5 fledglings, a rare phenomenon in The Netherlands (in a long-term study on the Veluwe, covering 537 nests in 1974-2005, only once before – in 1988 – 5 chicks on a single nest fledged, and this happened again in 2005, Photo 4). Few breeding birds in The Netherlands were in first-year plumage, i.e. 2.2% of 46 males and 4.8% of 122 females. Among surviving nestlings, sex ratio was highly male-biased in 2005: 410 males and 313 females on 255 nests. Since 1996, all nestlings were sex-identified on 2805 nests, resulting in a male-biased sex ratio of 55.1.0% (Table 12). Identified nest failures were mainly human-caused (Appendix 3). Persecution is widespread (Appendix 2), and apparently increasing in intensity. Sparrowhawk Accipiter nisus: mean onset of laying was 30 April (range 11 April-28 May, N=199; Appendix 3), with small regional variations (Table 13). Over the period 1984-2005, onset of laying did not show a systematic change (Fig. 5). Mean clutch size was 4.85 (N=182, Appendix 4), mean number of fledglings/successful pair 4.03 (N=234, Appendix 5). The secondary sex ratio did not differ from unity (Table 14: 333 males, 340 females, 161 nests), as found in the long-term average of 50.7% (2294 nests, 8643 nestlings). Common Buzzard Buteo buteo: on average, egg laying started on 6 April (range 9 March-20 May, the tail consisting of repeat layings, N=628; Appendix 3). Mean clutch size was 2.71 (N=537, with 40 C/4s and 1 C/5 and C/6 each; the latter presumably produced by two females; Appendix 4, Photo 5), mean brood size of successful pairs was 2.0 (N=1067; Appendix 5). A high proportion of clutches with 4 eggs is indicative of years with vole peaks, as in 1996, 1999, 2001 and 2005 (Fig. 7). On the other hand, clutches of 5 eggs are rare and occur I -2 times each year, irrespective of vole peaks and lows. Regional variations in onset of laying, clutch size and brood size were probably linked with vole abundance throughout the season (Table 15). Most regions showed starvation of nestlings, indicating declining vole availability later in the breeding cycle. Food choice was highly versatile, with 57 bird species, 18 mammal species, 7 species of reptiles and amphibians, and some fishes (Appendix 11, N=1526). Common voles Microtus arvalis are probably underrecorded (17.4% 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 11), The secondary sex ratio was female-biased: 306 females on 599 nestlings in 288 nests. Over 1996-2005, sex ratio was male-biased, though (52.2% in 4285 nestlings, 2180 nests; Table 16). Out of 104 identified causes of failure, 70 could be attributed to deliberate human action (Appendix 2). In 2005, two nests were recorded on electricity pylons (4 such cases recorded in 2003), and one nest was found on the ground in the same spot as in 2003 (but failed). All these cases were observed in open farmland in the western and northern Netherlands, where trees suitable for nesting are scarce. Osprey Pandion haliaetus: nesting attempts in the Oostvaardersplassen were not recorded (Frank de Roder). During summer, solitary Ospreys were observed on the northern Veluwe, but nesting did not occur (Holmer Vonk). An artificial nest on an electricity pylon in the central Netherlands was used as a plucking post by a Germanringed bird in August-October 2005 (www.rekel.nl). Eurasian Kestrel Falco tinnunculus: onset of laying averaged 30 April (range 23 March-11 June, N=582; Appendix 3). Mean clutch size was 5.14 (N=547; Appendix 4), mean number of fledglings/successful nest 4.30 (N=755, Appendix 5). Compared with most of the country, Groningen (NE-Netherlands), Zeeland and Zuid-Holland (western and SW-Netherlands) had a rather poor breeding success (Table 17). Laying date and clutch size showed wide fluctuations over the years, largely in synchrony between regions and suggesting parallel fluctuations in vole numbers (Figs. 9, 10). Recently, this synchrony seems to have dissolved (especially since the last couple of years). Eurasian Hobby Falco subbuteo: mean start of laying was 9 June (range 29 May-20 June, only 3 in May, N=39; Appendix 3). Mean clutch size was 2.87 (N=16; Appendix 4), mean number of fledglings/successful nest 2.29 (N=69; Appendix 5). Most pairs were recorded in open farmland where they nested in crow’s nests as high as possible in – mainly – electricity pylons and poplars (resp. 16x and 25x, out of a total of 74 nests). Secondary sex ratio in 12 nests was in favour of females; 11 males and 15 females (sexes identified by body mass and vocalisations). In 139 nests with 331 chicks in 1996-2005, a slight female-bias emerges but with large differences between years (due to small sample sizes; Table 19). Peregrine Falcon Falco peregrinus: after a decade of increase, the population slightly declined (24 territorial pairs, compared with 26 in 2004; Table 20-21). Some non-breeding birds were also recorded. Overall, 36 nestlings fledged (14 males, 14 females, 8 sex unknown; Table 20). Mean onset of laying was 22 March, ranging from 3 March through 24 April (Appendix 3). Mean clutch size was 3.3 (N=12; Table 20), mean number of fledglings/successful pair 2.4 (N=15; Table 20). Breeding of Carrion Crow, Magpie and Woodpigeon was recorded in close association with Peregrines (as in Eurasian Kestrel and Eurasian Hobby).