Trends en broedresultaten van roofvogels in Nederland in 1999

A total of 4215 nest record cards covering 1999 was received, comprising 10 raptor species (Appendix 1) and covering almost all regions and habitat types (Fig. 1). Only basic data are presented here. From these cards, clutch size was determined (only completed clutches used), brood size (usually during ringing or afterwards but before fledging and based on inspection of the nest cup), sex ratio (based on nests where all surviving young at ringing age were sex-identified and measured/weighed) and onset of laying (mostly back-calculated from wing length and controlling for clutch size). Food items were collected during nest visits (Appendix 9 and 10 for Goshawk and Common Buzzard respectively), but these data are biased towards large prey items given the timing of most nest visits (late nestling stage). During 1999, at least 8792 nestlings of ten raptor species were ringed (not all data received by late December). Weather conditions during the winter of 1998/99 were mild, spring and summer conditions excellent. 1999 proved to be a peak in the numbers of common voles Microlus arvalis, but numbers of rabbits Oryctolagus cuniculus were (and have been for a number of years) poor. Social wasps were not particularly abundant. Honey Buzzard Pernis apivorus: mean onset of laying was 24 May (range 16 May-3 June, n=19, Appendix 2), with little variation between regions (Table 2), a mean clutch size of 2.0 (Appendix 3) and mean number of fledglings/successful pair of 1.9 (Appendix 4). In 27 out of 36 nests eggs were laid, resulting in 17 successful nests. Causes of failure were mainly predation: 2x during egg stage, 2x of nestlings and 2x of adult breeding birds, if identified by Goshawks. Marsh Harrier Circus aeruginosus: mean onset of laying was 23 April (n=148, range 3 April-22 May; Appendix 2), mean clutch size 4.8 (n=99, Appendix 3) and mean brood size 3,3 (n=158, Appendix 4). Earliest start of breeding was recorded in the warmer southwestern Netherlands; Table 3). Secondary sex ratio in 118 nests was 56.7% (220 males, 168 females). Hen Harrier Circus cyaneus: lingering population mainly restricted to Wadden Sea Islands, where especially reproduction has been poor during the last decade. Mean onset of laying was 4 May (13 April-22 May, Appendix 2), mean clutch size 4.4 (n=7. Appendix 3) and mean brood size 2.4 (n=11, Appendix 4), with small variations per Wadden Sea Island (Table 4). Secondary sex ratio in 10 nests was 50% (12 males, 12 females). Preliminary results from an analysis of breeding data and survival statistics from ringing recoveries indicate that the poor performance in The Netherlands is not due to impoverished survival probabilities, but probably due to poor breeding results, the latter most likely caused by vegetation succession (more herbs and shrubs) and decreasing prey availability (Marjolein Lof, Frank van den Bosch). Montagu’s Harrier Circus pygargus: a detailed overview of last year’s results is published elsewhere in this Takkeling (cf Koks & Visser). The Dutch population counted 34 pairs, mainly restricted to farmland (especially alfalfa) in the province of Groningen. Breeding success is dependent on active nest protection, which is a success in the Netherlands because of the cooperation between farmers and raptorphiles. Mean onset of laying was 20 May (Appendix 2), mean clutch size 3.7 (Appendix 3) and mean brood size 2.8 (Appendix 4). Breeding results were comparatively good following a vole peak. Secondary sex ratio in 11 nests was 50% (18 males, 18 females). Goshawk Accipiter gentilis: the range expansion in the Netherlands in the last three decades has been spectacular (Fig. 2), with 210 squares of 5x5 km occupied in 1973-77 (500-600 pairs, mainly woodland on sandy soil in the eastern Netherlands), 578 squares occupied in 1978-88 (1200-1400 pairs, spreading westwards), 754 squares occupied in 1989-92 (1700-2000 pairs, colonising open farmland with small woodlots in the river district, recently planted polders in Flevoland and forested dunes along the coast) and 914 squares occupied in 1995-99 (1800 pairs, even breeding in open farmland in the northern and western Netherlands, except for the province of Zeeland). The range expansion was accompanied by some decrease in former strongholds in the eastern Netherlands, presumably because of a serious decline in prey biomass (very strong decline of woodpigeons, but also several other prey species). The occupation of the western Netherlands is noteworthy in respect of range, although the number of breeding pairs per square remains small (between 1 and 3); this low density is mainly due to lack of sufficient nesting sites and the density of the human population (but resilience of Goshawks towards humans is remarkable, some pairs even breeding in city parks). Mean onset of laying was 1 April (n=338, range 15 March-29 April, Appendix 2), with some regional variation caused by differences in spring temperatures: southern provinces are generally slightly warmer in March and therefore show earlier onset of laying by 2-5 days; Table 5). Mean clutch size was 3.5 (n=241, Appendix 3), mean brood size 2,9 (n=402, Appendix 4). Secondary sex ratio in 310 nests again showed a preponderance of males (50.7%, Table 6), but less so than in previous years. Prey remains found on and near nests are summarised in Appendix 9: racing pigeons, woodpigeons, corvids and starlings are most often captured but regional variations occur relative to prey availability. Sparrowhawk Accipiter nisus: this species also showed a clear increase since the 1970s, with 425 5-km squares occupied in 1973-77 (1200 pairs), 683 squares in 1978-88 (2000-2500 pairs), 860 squares in 1989-92 (3400-4000 pairs) and 1054 squares in 1995-99 (3900-5000 pairs). Presently, even very open farmland in the northern and western Netherlands is occupied, including urbanised regions as Amsterdam, Rotterdam and bordering towns and cities (Fig. 3). Former strongholds in the eastern Netherlands (woodland dominated by Goshawk and Common Buzzard) show -at least locally- severe depletion of the breeding population, poor breeding success (predation!) and high turnover among breeding birds (ditto). This decline is off-set by a strong increase in the western Netherlands, where the species thrives. Mean onset of laying was 29 April (n=323. Appendix 2), mean clutch size 4.8 (n=260. Appendix 3) and mean brood size 4.0 (n=336, Appendix 4), Small regional variations were recorded in start of laying, clutch size and brood size (Table 7). Secondary sex ratio was determined in 274 nests: 546 males and 556 females (Table 8). Seasonal changes in sex ratio, based on 3163 nestlings in 1997- 99, are shown in Table 9. Common Buzzard Buteo buteo: Common Buzzards have shown a most spectacular range expansion during the last three decades, with 630 5-km squares occupied in 1973-77 (1650 pairs), 958 squares in 1989-92 (5000-6000 pairs) and 1317 squares in 1995-99 (8.000-10.000 pairs)(c/ Fig. 4). In the 1970s, the species was almost entirely restricted to woodland on sandy soil in the eastern and southern Netherlands, increasing towards the west and into wetlands and farmland in the 1980s. It is thought that three successive vole peaks (from mid-1988 onwards) caused a very high production of young (see Appendix 7 for annual production figures in Drenthe and Flevoland in 1984-99), whose survival was enhanced by three successive very mild winters (1987/88-89/90). These birds probably entered the breeding population in the second half of the 1990s, causing another pronounced upsurge in breeding numbers, this time particularly apparent in the western Netherlands. Presently, Common Buzzards can be found breeding wherever there are trees (and sometimes even when there are none, i.e. on electricity towers), from dense woodland to sparsely wooded farmland, from marshland to extremely dry habitats, in cities and industrial sites, in recreation parks and along highways. It has become the commenest raptor of The Netherlands, by far. Mean onset of laying 4 April (n=931, Appendix 2), mean clutch size 2.7 (n=576, Appendix 3) and mean brood size 2.3 (n=1142, Appendix 4). These figures are typical of vole peak years (as also in 1996, cf. Appendices). Regional variations in start of laying, clutch size and brood size were small (Table 10); retarded laying dates were recorded in areas with poor food availability, like the Veluwe area (coarse sandy soil, poor pine woodland, low densities of voles, mice and rabbits). Four repeat layings started on average on 21 April (range 18-24 April), Secondary sex ratio in 1999 in 312 nests was 49.4% (346 males, 354 females; Table 11), Combined for 1997-99 (790 nests, 1616 nestlings), sex ratio showed a slight seasonal shift with males becoming progressively more common towards the end of the laying season (Table 12). The prey list varied from birds to small mammals (up to hare size), even hedgehogs, fawns of roe deer -probably found at fox dens-, muskrats, a wide variety of birds (mainly nestlings or juveniles), amphibians, snakes and fishes (Table 10). Moles, common voles, rabbits, starlings and racing pigeons were found most often (biased towards large prey items because most nests were visited in the second half of the nestling stage). Rabbits were always small to medium-sized young, with a mean length of hind foot of 59.4 mm (SD=7.8, n=60, variation 50- 85 mm). Of 22 hares, mean hind foot length was 85.1 mm (SD=19.6, variation 41-118 ram), also indicating a preference for small young and only exceptionally larger young. Kestrel Falco tinnunculus: mean onset of laying was 25 April (n=575. Appendix 2), mean clutch size 5.1 (n=484. Appendix 3) and mean brood size 4,3 (n=721, Appendix 4). Regional variations may indicate small sample sizes and variations in local food supply (Table 13). Kestrels using nestboxes had a better nesting success (668 out of 731 successful) than birds using natural nests (31out of 37). Hobby Falco subbuteo: mean onset of laying was 9 June (n=58, Appendix 2), mean clutch size 2.8 (n=29, Appendix 3) and mean brood size 2.4 (n=87, Appendix 4), with only small regional variations (Table 14). Main nest supplier was the carrion crow (59x), with two nests of magpie and one of woodpigeon. The only disadvantage of crow’s nests is the frequent presence of nylon in the nest cup, which in three cases resulted in entangled and disabled nestlings. Most nest were situated in tall trees with good views, irrespective of tree species. Out of 78 nests identified, 13 were situated on crow’s nests in electricity towers. This may indicate a switch towards breeding in very open farmland, where depredations of Goshawk and Common Buzzard are less likely to occur. During 1996-99, 115 young were sexed (using measurements, weights and calls) in 46 nests: 57 males and 58 females (Table 15). Peregrine Falcon Falco peregrinus: the highest number ever found breeding in The Netherlands, i.e. 6 pairs. Five pairs were using specially designed nestboxes on industrial sites, another bird used an old crow’s nest on a electricity tower. Five out of six pairs were successful, raising 14 nestlings (6 males, 8 females) in four nests. Mean onset of laying was 15 March (n=5, range 8-29 March), clutch size 1x2, 1x3 and 2x 4, and brood size 1x 1, 1x 2, 1x 3 and 2x 4 nestlings. All nestlings were colour-ringed with yellow bands carrying an inscription in black.