Broedresultaten en trends van roofvogels in Nederland in 1997

Over 1997, 2571 nests comprising ten raptor species were located in The Netherlands and visited at least once during the breeding cycle, No breeding records were received of Black and Red Kite. Submitted nest cards show a preponderance of tree-nesting raptors breeding in woodland on pleistocene soils, mainly in northem and eastem Netherlands (compare Fig. 2 with Fig. 1). However, coverage of the Dutch population has considerably improved compared to 1996, with presently the following proportion of the Dutch raptor populations under scrutiny: Honey Buzzard 5%, Hen Harrier 6%, Montagu’s Harrier 100%, Marsh Harrier 13%, Goshawk 24%, Sparrowhawk 7%, Common Buzzard 15%, Kestrel 11 %, Hobby 10% (taking into account a population decline of at least 50% during the last decade) and Peregrine 100% (Appendix 1). Weather conditions in winter 1997/98 were severe in late December and early January, but improved thereafter. Late winter and spring weather was excellent, but conditions in late April, May and June were rather wet. During April, three cycles of severe frost (6-8, 13 and 21-24 April) caused havoc in wasp populations, which was exacerbated by wet conditions in May. July and August showed normal and excellent weather with high temperatures and little rainfall. Overall, food supply was extremely poor, with troughs in populations of voles (Microtus arvalis, Clethrionomys glareolus) and mice (Apodemus sylvaticus), poor reproductive performance in Rabbits and the lowest populations levels of wasps Vespula germanica and V. vulgaris recorded since at least 25 years. Bird numbers were not affected by severe winter weather, but reproductive output was depressed by adverse weather in April and May. During 1997,6730 nestlings often raptor species were ringed, i.e. almost 2000 less than in the vole-year of 1996 (Table 1). Much smaller numbers were evident in Honey Buzzard, Common Buzzard and Kestrel. Honey Buzzard Pernis apivorus: mean onset of laying was 27 May, ranging from 19 May to 7 June (SD=4.9, N=20; Appendix 2). Clutch size was 1x 1 and 21x 2 eggs (Appendix 3). Number of fledglings/successful pair was 9x 1 and 7x 2 (Appendix 4). Breeding success was low as a result of very low wasp numbers. In two well-studied areas in Drenthe, the number of occupied territories was smaller than usual, with fewer pairs starting egg-laying and only 1 out of 13 pairs succeeding in raising a single fledgling (Tables 2 and 3). Pairs tried to substitute failing wasp availability by depredating bumblebee nests, but to no avail. Breeding success had never been so small in the period of 1973-97. Marsh Harrier Circus aeruginosus: mean onset of laying was 29 April (N=145, Appendix 2), being earliest in the warmer delta of SW-Netherlands and later in northern Netherlands (Table 4). Clutch size averaged 4.8 (N= 132, Appendix 3) and number of fledglings/successful pair 3.4 (N=149, Appendix 4), again with some regional variation (Table 4). Collections of prey remains on nests showed a wide variety of prey species, among which high numbers of birds (as a consequence of low vole numbers) and hares (Appendix 10). Secondary sex ratio in 119 nests was 211 males and 189 females.Causes of failure included desertion (1x), predation of eggs (4x) or nestlings (1x), adverse weather (lx) and human interference (6x). Hen Harrier Circus cyaneus: four pairs started on average on 29 April with egg-laying (Appendix 2). Clutch size (mean 3.3, SD=0.7,N=6) and number of fledglings/successful pair (3.2, SD=0.5,N=5) were small. Secondary sex ratio in 5 completely sexed nests was 9 males and 7 females. Reproductive output has become very small in the Dutch population, with a large proportion of pairs not laying or failing early in the breeding cycle, and the remaining pairs fledging few young. Presently, a study is undertaken to analyse the available data from the main breeding grounds, the Wadden Sea Islands. Montagu’s Harrier Circus pygargus: a full survey revealed the presence of 32 pairs in The Netherlands, restricted to Groningen (21), Lauwersmeer (7) and Flevoland (4). A minimum of 36 nestlings reached fledgling status. Mean onset of laying was 24 May (N=15, Appendix 2), mean clutch size 3.6 (N=10, Appendix 3) and mean number of fledglings/successful pair 2.2 (N=14, Appendix 4). A single case of bigamy was noticed (an adult male raising 2x 3 young). Secondary sex ratio in 11 nests was 12 males against 11 females. Most pairs nested in farmland: 13x winter wheat, 6x alfalfa, possibly once in oil seed rape and 7x in semi-natural reed/rough herbage. Nests in farmland were protected in cooperation with farmers. Despite low vole numbers, breeding success was slightly better than in 1996. Goshawk Accipiter gentilis: population stable (mostly), declining (in strongholds without persecution) or slightly increasing (fringes of breeding range, with first breeding case on one of the Wadden Sea Island: Texel, successful nests with 2 young). Breeding birds almost without exception in adult plumage; 85 out of 86 males and 186 out of 203 females in a country-wide sample. In a detailed study of a stable population in western Drenthe. the major part of the breeding birds in 1990-97 was older than 4-5 years, and first-year breeding birds had become extremely scarce, even in years with a – temporary?- upsurge in numbers, as in 1996 (Table 5). In this area, as on the Veluwe, the available data suggest a decline in reproductive output, possibly because of senility among territorial birds, and an increase in sex ratio. Ironically, in areas where human persecution is rife, reproductive output is better and breeding densities are higher because of a higher turnover; recruitment of surplus birds (mostly > 1 year old) guarantees a higher production of fledglings, unless persecution concentrates on disturbing nests and is very intense. Mean onset of laying was 3 April (N=335, Appendix 2), i.e. earlier than in 1996 because of milder temperatures in late winter (see also Appendix 5 for long-term data). Regional variation in laying dates suggests a slightly earlier onset on laying in the southern Netherlands (Noord-Brabant, Limburg), probably also a result of local variations in climate (Table 6). Clutch size averaged 3.1 (N=233, Appendix 3) and number of nestlings/successful pair 2.6 (N=392, Appendix 4). Secondary sex ratio in 335 nests was 56.5%, i.e. 493 males and 379 females (Table 7). Causes of failure were human-caused (22x, among which poisoning, egg collecting, taking of young, nest destruction, shooting) or natural (16x desertion, 4x egg predation, 1 x young predation, 3x nest destroyed by high winds). Human-caused failures are probably much more prominent, but often difficult to prove beyond doubt. Prey remains found on and near nest showed pigeons (Feral and Wood), thrushes, corvids (mainly Jay), Starling and Rabbit to be important food sources. Impact of Goshawks on Long-eared Owl is probably serious (Appendix 11). Sparrowhawk, Accipiter nisus: mean onset of laying varied regionally from 27 April to 10 May (Table 8) and averaged 1 May (N=147, Appendix 2). Both clutch size (mean 4.50, N=188; Appendix 3) and number of fledglings/successful pair (mean 3.6, N=272; Appendix 4) were smaller than in 1996. Longterm data indicate that breeding results in 1997 were smaller than ever (Appendix 6). This was also apparent in the high incidence of nest losses, including desertion (12x), egg predation (12x), depredation of nestlings (16x, mainly by Goshawk) and predation of territorial birds (3x, by Goshawk). Human-caused failures included egg collecting (1x), taking of nestlings (1x), nest disturbance (1x) and cutting down the nest tree (1x). Secondary sex ratio was established in 245 nests: 50.2%, i.e. 450 males and 446 females (Table 9). The majority of breeding birds consists of adults: 42 out of 44 males and 107 out of 122 females. Chances of settlement as yearling are better in areas with high rates of Goshawk predation than elsewhere because of faster turnover. Common Buzzard Buteo buteo: troughs in vole and mice populations became visible in prey remains found on nests, with a high frequency of birds (49 species, covering 32% of all prey items; Appendix 12), and important numbers of Mole, Rabbit and Hare. Mean onset of laying was 6 April (N=632, Appendix 2), mean clutch size 2,2 (N=388, with only 2 C/4 and 1 C/5; Appendix 3) and mean number of fledglings/successful pair 1.9 (N=762, with only 4 nests with 4 nestlings; Appendix 4). Long-term data showed that breeding performance in 1997 was exceptionally poor (Appendix 7), mainly caused by food shortage. Causes of failure were human-induced (4x), desertion (12x), egg predation (lOx), nestling predation (16x, see also Photo) and predation of breeding birds (lx). This does not include nonlaying, which was a prominent feature of the 1997-breeding season. Secondary sex ratio was established in 172 nests with nestlings of >29 days old: 49.5%, i.e. 152 males and 155 females. Kestrel Falco tinnunculus: mean onset of laying in 1997 was two weeks later than in 1996, i.e. 6 May (N=371, Appendix 2), showing the impact of poor vole numbers. This was apparent in the entire country (Table 12). Long-term data for Groningen showed that 1997 ranks among the years with poorest breeding performance (Appendix 8), Clutch size averaged 4.8 (N=367, with only 9 C/7; Appendix 2) and number of fledglings/successful pair 4.0 (N=457, with only 2 nests having 7 fledglings; Appendix 4), Causes of failure were human-induced (2x), desertion 19x), egg predation (4x), nestling predation (2x), predation or death of adults (3x) and inferior nest (1x). Secondary sex ratio in 129 nests was 41.5%, i.e. 194 males and 273 females. Hobby Falco subbuteo: as compared to the 1970s, Dutch Hobbies hardly ever start laying in May anymore, in 1997 for example only 2 out of 50 pairs. Onset of laying in 50 pairs averaged 12 June, with several pairs even laying in late June and early July (Table 13, Appendix 2). Mean clutch size was 2.8 (N=27, Appendix 3), mean number of fledglings/successful pair 2.4 (N=67, Appendix 4). Secondary sex ratio in 12 nests was 53.3%, i.e. 16 males and 14 females. All breeding birds were in adult plumage (15x males, 16x females), and sightings of yearlings at breeding sites and elsewhere have become rare. Causes of failure were desertion (1x), egg predation (1x) and nestling predation (2x). The Dutch population has more than halved during the last decade, with presently only 800 pairs left at most. Former strongholds show clear declines, locally to the point of extinction. Peregrine Falco peregrinus: 3 pairs raised a total of 9 fledglings (4 males, 5 females), using specially designed nestboxes provided at industrial buildings. All nestlings were colour-ringed (orange). Start of laying was 3 March, 12 March and 20 April, with 2x a clutch of four. Another pair settled at another power plant, but did not yet breed.