Trends en broedresultaten van roofvogels in Nederland in 1998

A total of 3568 nest cards were collected during 1998, comprising ten raptor species (Appendix 1) and covering large parts of the country (Figure 1). Clutch size is calculated for completed clutches only, number of fledglings is based on the number of young encountered during the last nest visit (usually during ringing; nests with young counted from the ground are omitted as these are liable to errors) and secondary sex ratio is based on nests where all surviving young were sexed according to diagnostic measurements described in Bijlsma (1997), Onset of laying is directly observed (nests controlled during laying) or (most often) back-calculated from the age of the oldest nestling (based on wing length) and species-specific incubation periods (controlling for clutch-size, cf. Bijlsma 1997). Weather conditions during the winter of 1997/98 were very mild (Ijnsen frost index of 4.6, on a scale of 0-100). Spring conditions were excellent, but the summer was extremely wet. Food supply had improved somewhat as compared to the very poor conditions in 1997, especially regarding voles and mice, but not Rabbits. Social wasps, especially Vespula vulgaris, had an exceptionally good winter survival, resulting in moderate numbers of nests during summer. During 1998, 8507 nestlings of ten raptor species were ringed (Table 1). Honey Buzzard Pernis apivorus: mean onset of laying was 27 May, ranging from 16 May through 16 June (N=22, Appendix 2). Mean clutch size was 2 eggs (N=15), the mean number of fledglings/successful pair 1.6 (10x 1,13 x 2; Appendix 4). The latter figure is biased, as mortality among nestlings has become rather high in recent years, mainly because of increased predation by Goshawks. This predation often goes unrecorded when no further nest visits are carried out after ringing. Nevertheless, predation of nestlings and/or adults was recorded in 8 out of 44 nests, a minimum considering the fact that frequent nest visits (also after fledging) were restricted to three study areas in The Netherlands (covering 25 nests). Marsh Harrier Circus aeruginosus: local declines (Lauwersmeer, Oostvaardersplassen, marshes in central Netherlands) are off-set by regional increases (Wadden Sea Islands, provinces of Groningen, Noord-Holland and Zeeland). Mean onset of laying was 26 April (N=136, Appendix 2) with some variation between regions (Table 2), mean clutch size was 4.9 (N=104, Appendix 3), mean number of fledglings 3.4 (N=141, Appendix 4). Secondary sex ratio in 108 nests was 55.6% (203 males and 162 females). Causes of failure were often human-caused (8x), as compared to predation (3x) and other natural causes (3x). Prey remains on nests with nestlings (N=92) consisted mainly of j Phasianus colchicus (18). Oryctolagus cuniculus (15, mean length of hind foot 58.8 mm). Sturnus vulgaris (14)and Lepus europaeus (12), Hen Harrier Circus cyaneus: almost entirely restricted to Wadden Sea Islands, where population shows decline and reproduction is poor. On Ameland, only 4 out of 13 pairs raised a total of 9 fledglings, showing a distinct decline from the 22-26 pairs in 1990-93. Six nests on Texel had a mean clutch size of 3,8 eggs (Appendix 3), and a mean number of fledglings/successful pair of 2.8 (Appendix 4). Mean onset of laying was 2 May (N=B, Appendix 2). Secondary sex ratio on 4 nests was 4 males and 7 females. It is thought that prey availability has declined, as a result of an increase in shrubs and rough herbage on Wadden Sea Islands. However, competition with the strongly increased population of Marsh Harriers can not be excluded either. Montagu’s Harrier Circus pygargus: a country wide survey revealed 28 territories, mainly in NE- Groningen (19), Lauwersmeer (2, probably under-recorded), Zuidelijk Flevoland (6) and NW-Groningen (1), i.e. quite similar to 1996 (N=23) and 1997 (N=32). Mean clutch size was 3.9 (N=lo, Appendix 3), mean number of fledglings 2.6 (N=14, Appendix 4), with a secondary sex ratio of 14 males and 14 females (N=l 1 nests). Onset of laying was 20 May (N=13, Appendix 2). Most nests in Groningen were situated in alfalfa (6) and winter wheat (6); very few nests are nowadays built in natural vegetation as reedbeds (2 in Lauwersmeer). Nests in farmland are protected in cooperation with farmers and harvesters. A minimum of 37 nestlings fledged, a good result given the appaling weather conditions in June and July. Goshawk Accipiter gentilis: bred for the first time on the Wadden Sea Islands of Vlieland and Schiermonnikoog (1 pair each in 1998; it bred for the first time on Texel in 1997), colonised open farmland and cities (5 pairs in Amsterdam) and increased its distribution with some 60 5x5 kmsquares since 1995 (mainly in farmland and dunes of western and northern Netherlands; some 90 extra pairs involved). However, overall the population is declining, mainly in its core breeding area in woodland in the eastern Netherlands. The latter is thought to be due to a dramatic decrease in prey biomass, especially of prey species weighing 100-500 g. Woodpigeon numbers in woodland on sandy soils have been decimated since the mid-1970s (following conversion of cereal growing into green maize), and the availability and/or numbers of racing pigeons also declined (as evident from decline in number of pigeons rings found beneath Goshawk nests, and declining importance of this species in prey lists of Goshawks). Many other prey species in the eastern Netherlands have also declined, such as Turtle Dove, Cuckoo, Long-eared Owl, Magpie, Jackdaw, Carrion Crow, Starling and Rabbit. Although still important, racing pigeons have declined in importance as summer prey (25% of 793 prey items collected in summer of 1998, compared to >60% in 19705). Other important prey species were Jay, Starling and Woodpigeon (Appendix 9). Mean onset of laying was 4 April (N=323, Appendix 2), with the earliest ever Goshawk starting on 11 March, and some regional variation (Table 3). Goshawks breeding in woodland show a later onset of laying than those breeding and feeding in farmland, probably as a result of impoverished food availability conditions in woodland. Mean clutch size was 3.3 (N=2o9, Appendix 3), and mean number of fledglings was 2.7 (N=369, Appendix 4). Clutch and brood size declined with progressing season. Secondary sex ratio at 307 nests was 55.1% (456 males and 371 females)(Table 4). In 39 nests, the cause of failure was deduced, i.e. 22x man-induced and 17x natural (mainly desertion of eggs). Sparrowhawk Accipiter nisus: declines in coniferous woodland and dunes (mainly result of Goshawk predation) are balanced by increases in farmland and cities (for example, >2O pairs in Amsterdam). Mean onset of laying was 1 May (n=376. Appendix 2), late-laying pairs indicative of repeat layings. Mean clutch size (4.7, N=298, Appendix 3) and mean number of fledglings/successful nest (3.9, N=362, Appendix 4) were very similar to those in 1996-97. Secondary sex ratio in 325 nests was 50% (640 males, 637 females). Over the years 1984-91 (Drenthe) and 1996-98 (The Netherlands) a secondary sex ratio of 2176 males and 2157 females was found. Causes of failure were man-induced (1 lx) and natural (desertion during egg stage 13x, egg predation 17x, nestling predation 45x, parent predation 2x and collapse of nest 4x). Goshawk predation is particularly severe at breeding sites in coniferous woodland and in the dunes, and is responsible for regional declines. Buzzard Buteo buteo: by far the most successful raptor in The Netherlands, with presently >8000 pairs. Increase noticed in core areas (coniferous woodland on sandy soil), but especially in farmland, marshes and built-up areas. Colonised hitherto empty quarters like the province of Zeeland, newly planted forests in Flevoland, dunes and farmland in the western Netherlands, marshland and very open arable land and grassland in the northern Netherlands, Of the Wadden Sea Islands, Texel, Vlieland and Terschelling had been occupied for some years; a further increase occurred in 1998 when Ameland (2 nests) and Schiermonnikoog (1 nest) became also occupied. Mean onset of laying was 6 April (N=835, Appendix 2), showing some regional variation (Table 6). Mean clutch size was 2.5 (N=49l); a clutch of 6 eggs was laid by two females (not included in Appendix 3). Mean number of fledglings/successful pair was 2.0 (N=983, Appendix 4), probably slightly biased as nestling mortality was rather high in June following inclement weather and many ringers stop visiting nests after having ringed the young. Clutch and brood size showed a clear seasonal decline. Secondary sex ratio at 285 nests was 52.5% (298 males, 270 females). Combining all nests with sexed young over 1990-98 (N=551), the sex ratio is male-biased (51.8%, i.e. 552 males and 514 females). Among causes of failure, human-caused failures dominated (25x disturbance, 20x eggs taken, 2x young taken, 4x destruction of eggs, 1x poisoned, 3x shot, 8x nesting tree cut down), as compared to natural causes (10x desertion, 5x egg predation, lOx nestling predation, 6x inclement weather, 1x mortality of parent). The frequency with which nestlings are depredated by Goshawks is increasing, apparently as a result of food shortage experienced by Goshawks breeding in coniferous woodland on sandy soil (avian prey biomass declined dramatically during last two decades). Buzzards are versatile raptors, as shown by prey items found on and near nests in 1998 (Appendix 9): 49 bird species, 18 mammals, 5 reptiles, 3 amphibians and 3 fishes. Most important prey species are (in declining frequency of abundance) Mole, Common Vole, Rabbit and Starling. Rabbits taken as prey are medium-sized, as deduced from the mean length of hind feet of 60.6 mm (N=24, range 48-71). This is the same size class as taken by Marsh Harriers (mean 58.8 mm, N=B) but much smaller than those taken by Goshawks (mean 66.8 mm, N=6, range 55-88 m). The same applies to Hares: mean hind foot length when taken by Buzzards is 98.8 mm (N=l4, range 68-119 mm), and 125.8 mm (N=4, range 104-143 mm) when taken by Goshawks. Kestrel Falco tinnunculus: sharp decline in The Netherlands since booming years of 1988-90, with presently maybe only 4000 pairs (from a maximum of 6600-7700 in 1990, cf. Table 9). Long laying period (27 March through 4 July), with a mean onset of 27 April (N=502, Appendix 2). Mean clutch size was 5.2 (N=440, Appendix 3), mean number of fledglings 4.5 (N=621, Appendix 4). A clutch of 9 may have been laid by 2 females, although this was not evident from the eggs. Main causes of failure were desertion (17x), egg predation (6x), nestling predation (1x), starvation (2x, probably parents dead), death of parent (s)(2x) and nest collapse (1x). Hobby Falco subbuteo: slight increase in 1998, as compared to heavily depleted population figures of last decade. Numbers have declined to 750 pairs, from a maximum of 1700-2100 in 1985-92 (Table 9). Formerly densely occupied breeding areas in woodland are nowadays almost deserted, whereas farmland may have increased in importance. Mean start of laying was 11 June (N=62, Appendix 2), mean clutch size 2.8 (N=28, Appendix 3), mean number of fledglings/successful nest 2.4 (N=82, Appendix 4). Secondary sex ratio in 17 nests was 22 males and 25 females. Causes of failure included desertion (1x), egg predation (1x), nestling predation (10x) and nest collapse (1x). Predation, mainly by Goshawk, has become an important factor. Peregrine Falco peregrinus: increase in 1998 to 5 nesting pairs, one of which failed in the egg stage (dutch of 3 eggs). The remaining 4 pairs raised 8 nestlings (3 male, 5 female) which were colour-ringed. Except for a nest on a pylon (old crow’s nest), all pairs used nest boxes erected on industrial buildings. Onset of laying varied between 9 March and 30 April (Appendix 2), clutch size was 2x 3 and 1x 4 eggs. Trends of Dutch raptors since mid-1970s: all raptor species showed distinct increases throughout the 1970s and 1980s, partly a recovery from pesticide-induced losses in the 1960s, partly an adaptation to human-dominated habitats and planting of forests. However, in the 1990s many trends have been reversed, especially in Hen Harrier (range contraction, declining breeding success and ditto numbers), Goshawk (locally declining breeding success and ditto numbers in coniferous woodland, following steep decline in prey biomass and density-dependent regulation), Kestrel (population almost halved since 1990, having virtually disappeared from well-wooded areas on sandy soil, and very much relying on the provision of nest boxes) and Hobby (population more than halved, mainly following predation by Goshawks). Montagu’s Harriers barely manage to survive in open farmland; without intensive protection by farmers and raptorphiles, the chances of survival are slim. Sparrowhawks show mixed fortunes, increasing in farmland and built-up areas (including cities) but decreasing in well-wooded regions (result of Goshawk predation). More or less stabilised populations occur in Honey Buzzard and Marsh Harrier, despite local variations. Only Buzzard and Peregrine are still increasing. Buzzards by far being the most common raptor in The Netherlands and Peregrines profiting from nest boxes provided at industrial buildings and population increases in Germany and Belgium. Red and Black Kites were, and still are, accidental breeding birds (not recorded in 1998).