Trends en broedresultaten van roofvogels in Nederland in 2003

Over 3500 nest cards of raptors were submitted in 2003 (Appendix 1), covering 11 species. The weather in 2003 was on average warm, sunny and extremely dry, the summer index being 81.7 (a very warm summer, based on temperatures in May-August). The frost index of the past winter was 9.4 (mild winter, based on temperatures in November-March 2002/2003). Many prey species registered low numbers in 2003 (as in 2002), especially voles and mice, rabbits (population has crashed in the 1990s, to levels <5% of the mid-1970s, partly following outbreaks of Rabbit Viral Haemorrhagic Disease) and hares. Bird numbers were average or below-average. An outbreak of the bird pest virus in spring resulted in bans on racing pigeon flights in spring/early summer, which particularly affected Goshawks. Social hymenoptera had a very poor year (only just index 2 on a scale of 1-5), despite a promising start in May (many prospecting queens). The extremely poor prey base was reflected in the much lower numbers of nestling raptors ringed (Table 1), mainly the result of fewer pairs producing eggs, smaller number of fledglings per successful pair, and higher losses among active nests. European Honey-buzzard Pernis apivorus: few pairs started breeding in 2003. In a well-studied region of West-Drenthe (45 km²), only 1 pair out of 9 is thought to have started egg-laying (single egg); this chick was killed by a juvenile Goshawk just before fledging. Poor food supply was probably the major cause of the failure to start laying, Overall in The Netherlands, clutch size was 2x 1 and 6x 2, brood size 6x 1 and 14x 2. Mean onset of laying was 31 May (22 May-9 June, SD=5.34, N=17), i.e. later than ever in 1996-2002 (range 22-29 May). Food shortage throughout the breeding season is the most likely cause of poor breeding performance in 2003, as also clearly seen from the small diameter of wasp combs collected on active Honey- buzzard nests (Fig. 1). Furthermore, 2 clutches were deserted (a rare phenomenon, though typical in years of short food supply), 1 clutch was depredated and 3 nests were depredated during the nestling stage (suggesting prolonged absence of parents). Of 23 nests checked, 16 were built by Honey-buzzards (of which 12 newly built), 3 by Goshawks, 2 by Buzzards, 1 by Sparrowhawk and 1 by Carrion Crow. Eurasian Marsh Harrier Circus aeruginosus: mean onset of laying was 25 April (range 29 March-31 May; Appendix 2, Table 3). Clutch size averaged 4.41 (SD=1.09, N=75; Appendix 3), mean number of fledglings/successful pair 3.10 (SD=1.09, N=88; Appendix 4). The number of active nests recorded was much lower than in previous years, suggesting an overall poor breeding performance in concert with poor food supply. All surviving nestlings in 42 nests were sexed: 48 males and 74 females. Over the years, a biased sex ratio has been prevalent (Table 4: 52.8% males in 697 nests with 2273 nestlings). Among prey remains found on nests, birds were particularly important (Table 5), partly caused by biased sampling (mostly late nestling stage) but also indicative of poor vole and ditto rabbit numbers. Hen Harrier Circus cyaneus: the steady decline of the Dutch population is ongoing to the present day. Reproductive output was exceedingly poor on all Wadden Sea Islands studied, with an average clutch size of 3.88 (range 2-6; Appendix 3) and an average brood size of 2.54 (range 1-5; Appendix 5). The Ameland population, for example, declined from 26 pairs in 1990 to 3 pairs in 2003 (of which 2 pairs fledged a total of 6 young). Onset of laying averaged 1 May (N=13, range 20 April-21 May). Montagu’s Harrier Circus pygargus: 35 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.grauwekiekendief.nl Northern Goshawk Accipiter gentilis: the trend of the past decade (declines in eastern and central Netherlands, expansion in the northern and western Netherlands, some increase in the southern Netherlands) is continuing to the present day. The colonisation of the Wadden Sea Islands, for example, has now reached Terschelling as well (first breeding in 2001, 3 pairs in 2003), suggesting a tweezer-like expansion from Noord-Holland to Texel (spreading eastwards), and via Lauwersmeer in northern Groningen/Friesland to Schiermonnikoog westwards (Table 6, Figure 2). The successive occupation of Wadden Sea Islands fits this dispersal pattern (Table 6), with presently only Ameland being devoid of breeding Northern Goshawks (but two summering birds in 2003, one of which a juvenile female). Regional variations in laying date may reflect variations in prey abundance; it is remarkable that early laying and large clutch size coincide with densely populated (Noord-Holland) and heavily eutrophied parts of The Netherlands (Noord-Brabant, Limburg; Table 8). Locally, Goshawk numbers have been declining since at least the early 1990s, as in western Drenthe (Table 7), stabilising at a lower density in recent years (and with indications of an increase in non-laying, smaller clutches and smaller number of fledglings/pair). Pairs fledging four chicks have become rare (Table 7, see also Appendix 4). This trend is caused by a serious decline in food-supply, especially of birds >75 grams and of rabbits (>95% decline since 1970s). In 2003, this situation was aggravated by an outbreak of bird pest in poultry, leading to the prohibition of racing with pigeons in spring and early summer (an important prey species during the breeding season; Appendix 9). On average, Dutch Goshawks started egg laying on 2 April (range 12 March-4 May. N=238; Appendix 2). Mean clutch size was 3.34 (SD=0.78, N=233; Appendix 3), mean number of fledglings/successful pair 2.52 (SD=0.88, N=315; Appendix 4). Among 34 identified nest failures, 22 were human caused (see also Bijlsma 2004). Nest take-overs by Egyptian Geese Alopochen aegyptiacus are recorded with increasing frequency, however, without impacting Goshawk density. Most breeding birds in The Netherlands were in adult plumage, i.e. 100% of 35 males and 91.6% of 154 females. Among surviving nestlings, sex ratio was highly male-biased in 2003: 386 males and 250 females on 211 nests. Since 1996, all nestlings were sex- identified on 2273 nests, resulting in a male-biased sex ratio of 55.0% (Table 9). Sparrowhawk Accipiter nisus: mean onset of laying was 29 April (15 April-31 May, N=197; Appendix 2), with distinct regional variations (Table 10). Mean clutch size was 4.78 (N=156, Appendix 3), mean number of fledglings/successful pair 3.78 (N=231, Appendix 4). The secondary sex ratio was slightly female-biased (Table 12: 334 males, 356 females, 180 nests). Adult pairs started laying earlier than pairs with one pair members in first-year plumage; also, clutch size and number of fledglings were higher. Pairs with both members in first-year plumage started laying two weeks later than adult pairs, and showed poor reproductive output (Tabel 11). Common Buzzard Buteo buteo: as in 2002, many pairs refrained from egg laying in 2003, presumably caused by food shortage (voles, mice, rabbits). On average, egg laying pairs started later than usual (8 April, N=628; Appendix 2), produced small clutches (mean 2.34, N=438, among which only 6 C/4 and -surprisingly – 1 C/5; Appendix 3) and few fledglings (mean 1,68/successful pair, N=832; Appendix 4). Other signs of food stress were recorded in the high incidence of starvation among nestlings and large within-nest age differences of chicks (as in 2002). Prey remains on nests showed a high proportion of birds, with a scattering of amphibians, reptiles, fishes and mammals (Appendix 10). However versatile, when voles and rabbits are in short supply, Common Buzzards face serious problems during food provisioning. The secondary sex ratio was highly male-biased: 63.0% of 230 nestlings on 138 nests. This is an artefact of the poor condition of nestlings in 2003, which made sexing of females in poor condition particularly difficult (and often impossible). Hence, many broods with nestlings showing masses and measurements intermediate between males and females remained unsexed. It is likely that many of these were low-weight females, resulting in a male-biased (and non-representative) sex ratio. The spread into more open habitats in the northern and western Netherlands is accompanied by an increasing frequency of nesting in electricity pylons (4 such cases recorded in 2003), as well as sporadic breeding on the ground (1 case in 2003). Osprey Pandion haliaetus: the artificial nest in the Oostvaardersplassen, erected in the same tree as where an adult pair had built a nest in 2002 (which was demolished during an October-storm), attracted a ringed male (left alu) in April 2003, afterwards followed by a female. The nest was partly improved during early summer, but then abandoned. The pair refrained from further breeding activities. Eurasian Kestrel Falco tinnunculus: many pairs did not lay, as shown by the poor occupation rate of nest boxes. Onset of laying averaged 28 April, ranging from 29 March through 1 June (N=380; Appendix 2). Mean clutch size was 4.84 (N=358; Appendix 3), mean number of fledglings/successful nest 4.03 (N=520, Appendix 4). This poor breeding performance was recorded all over the country (Table 15), and resulted from a country-wide low in vole numbers. Prey remains found in nest boxes showed few voles (Table 16), with recently fledged birds being the main substitute (especially starlings and thrushes). Most Kestrels recorded were breeding in nest boxes (95%), but the biased sampling effort does not allow statements on what proportion of the Dutch population is nowadays depending on nest boxes. Eurasian Hobby Falco subbuteo: mean start of laying was 9 June (range 29 May-23 June, N=37; Appendix 2). Mean clutch size was 2.34 (N=24, only 2 started in May, Appendix 3), mean number of fledglings/successful nest 2.33 (N=82; Appendix 4). The switch from forest-breeding to open-land-breeding has become very pronounced during the latest decade (breeding in Groningen, Friesland, Noord-Holland used to be scarce compared to Drenthe and Gelderland, but see present distribution in Table 17). Presently, most pairs are recorded in open farmland where the birds prefer nesting as high as possible in – mainly – electricity pylons and poplars (Table 19). Secondary sex ratio in 17 nests was in favour of females: 16 males and 23 females (sexes identified by body mass and vocalisations). Peregrine Falcon Falco peregrinus: in 2003, the Dutch population increased again, from 12 territorial pairs in 2002 to 17 pairs in 2003 (of which 13 pairs laid eggs: 10x in nest boxes, 2x on crow’s nests in electricity pylons and 1x on a chemical plant). Overall, 27 nestlings were raised to fledging (10 males and 14 females, based on nests where all fledglings were sex-identified; Table 20). Most nestlings were also colourringed. Mean onset of laying was 14 March, ranging from 28 February through 6 April. Mean clutch size was 3.8 (N=5; Table 20), mean number of fledglings/successful pair 3.0 (N=9; Table 20). Successful monitoring of the expanding population of Peregrines will become increasingly difficult, especially when the tendency to start breeding on electricity pylons continues.