Broedresultaten en trends van roofvogels in Nederland in 1996

In 1996, members of the Dutch Raptor Group located and checked 2382 nests of eleven raptor species in The Netherlands, mainly distributed in the northern and eastern parts of the country where woodland is concentrated and raptor densities are highest (compare Figure 1 and 2, Appendix 1). Apart from this combined effort, many specific studies are being carried out, the results of which will be published separately. For example, in 1996 147 nests of Sparrowhawks were closely monitored in 5 plots of 36- 240 km2 (totalling 626 km2). The weather in 1996 was on average cold, very dry and sunny (as compared to long-running means). Food supply abounded, following bumper crops of beechnuts and other seeds in autumn 1995. Peak numbers were recorded for Clethrionomys glareola, Microtus arvalis, M. agrestis, Micromys minutus and Apodemus sylvaticus. Equally, many passerines showed high survival rates during winter 1995/96. In 1996, 7735 nestlings often raptor species were ringed (Table 1), mainly Marsh Harriers, Goshawks, Sparrowhawks, Common Buzzards and Kestrels. The high ringing total as compared to 1995 is the result of an increased ringing effort and an improved reproductive output. Honey Buzzard Pernis apivorus: mean onset of laying in 34 pairs was 29 May (SD=4.9, range 20 May-9 June; Appendix 2 for further details). Clutch size was 1x 1 and 27x 2 (Appendix 3). Number of fledglings/successful pair was 6x 1 and 31x 2 (Appendix 4). All well-observed breeding birds were in adult plumage (20 males and 25 females). The population is probably stable, as found in 6880 ha of woodland in Drenthe with 10-11 territories in 1992-96. However, marked annual fluctuations in nest occupancy were found, with only 36-64% of the territorial pairs occupying a nest in 1992-96 (in which not always egg-laying commences), and only 18-50% of the pairs raising one or two nestlings (Table 2). Red Kite Milvus milvus: no territories located in 1996. Black Kite. M. migrans: first successful breeding case in The Netherlands in 1996: one fledgling (for further details, see De Takkeling 4,3: 15-20). Marsh Harrier Circus aeruginosus: mean onset of laying was 27 April (SD=8.9, N=114, range 11 April-26 May; see Appendix 2). Egg-laying was relatively late in the province of Groningen (as compared to 1991-95; Table 2), probably because of a collapse of the local vole population in early spring. Elsewhere in the Netherlands, egg-laying was on average at least a week earlier than in Groningen (Table 2). Clutch size was 3-8 eggs (mean 4.7, SD=1.0, N=85; Appendix 3), number of fledglings was on average 3.3 (SD=1.2, N=113, range 1-6; Appendix 4). Hen Harrier C. cyaneus: mainly restricted to Wadden Sea Islands (probably closed population, with recruitment only from locally produced young), where population decline is becoming apparent. On Ameland, for example, number of pairs in 1987-96 was resp. 15, 22, 22, 26, 22, 24, 24, 16, 15 and 15 (annually, only 9-12 pairs successfully raising young). This decline probably follows impaired reproduction (Table 5 for Ameland). Habitat changes and competition with Marsh Harriers may also play a role. In 1996, mean onset of laying was 12 May (Appendix 2), mean clutch size 3.8 (Appendix 3) and mean number of fledglings 2.4 (Appendix 4). A single pair was found on the mainland, nesting in a 15 m high stand of. Pseudotsuga menziesii in a large woodland (further details by Santing in De Takkeling 5,1). Montagu’s Harrier C. pygargus: full details supplied by Koks & Visser in De Takkeling 5/1. In 1996, 23 territories were located in The Netherlands, in which only 23 fledglings were raised. Mean onset of laying was 27 May (Appendix 2), mean clutch size 3.3 (Appendix 3) and mean number of fledglings 2.6 (Appendix 4). Th Dutch trend for 1990-96 is shown in Fig. 3. Special protection was necessary to prevent nests from being destroyed during harvesting. Goshawk Accipiter genlilis: stable population, with majority of breeding birds in adult plumage (98 out of 99 males, 154 out of 170 females). Mean onset of laying was 6 April (SD=8.0, N=216; Appendix 2), regionally ranging between 1 and 16 April (Table 6). Onset of laying correlates well with mean March temperature (Figure 4). Clutch size varied from 1 to 5 (mean 3.5, SD=0.8, N=177; Appendix 3) and mean number of fledglings was 2.8 (SD=1.0, N=289; Appendix 4). Only very slight differences in clutch and brood size were found between regions. Secondary sex ratio was determined in 240 nests where all fledglings were sexed: 365 males and 305 females. A preponderance of male nestlings is typical for most years and all regions in The Netherlands, as for example in Flevoland in 1989-96 (Table 7). Annual means (including SD, range and N) of onset of laying, clutch size and number of nestlings are supplied for three regions, i.e. Groningen (1990-96), Drenthe (1984-96) and Flevoland (1989-96)(Appendix 5). Sparrowhawk. Accipiter nisus: mean onset of laying was relatively early in 1996 (28 April, SD=9.0, N=158; Appendix 2, see also Appendix 6 for long-term data from several regions). Local variations in onset of laying, clutch size and number of fledglings may have been caused by climatic differences, variations in the proportion of first year breeding birds and ditto in predation pressure (especially by Goshawks)(Table 8). Mean clutch size was 4.8 (SD=1.0, N=131; Appendix 3), mean number of fledglings 4.0 (SD=1.2, N=197; Appendix 4). Secondary sex ratio in nests where all fledglings were sexed (N=174) was equal (357 males and 350 females). Common Buzzard Buteo buteo early onset of laying was registered in all provinces (Table 9), being on average 4 April (SD=7.2, N=571; Appendix 2). Given the severity of the preceding winter, a much later start of laying was expected (Table 11). Mean clutch size was 2.8 (SD=0.7, N=375; Appendix 3), indicating the effect of a good vole year (see Appendix 7 for long series of reproductive parameters in three provinces). Of three pairs with clutches with 5 eggs, only one succeeded in raising five fledglings, C/5's are exceedingly rare in The Netherlands, with only single ones found in Drenthe in respectively 1986 and 1989. Number of fledglings per successful pair averaged 2.3 (SD=0.8, N=748; Appendix 4). Based on a combination of body mass (taking into account crop contents), claw width, length of hind claw and tarsus width, reliably sexing of >30 days old nestlings (based on wing length) is possible (Bijlsma 1997). In 52 nests, all nestlings could be sexed, resulting in a sex ratio of 64 males and 55 females. The Dutch population consists almost entirely of adult breeding birds, with in 1996 only a single male (out of 293 aged as a breeding bird) and three females (out of 363 aged) in first year plumage. Hobby Falco subbuteo: mean onset of laying was 11 June (SD=6.8, N=23; Appendix 2), mean clutch size 3.2 (SD=0.4, N=10; Appendix 3) and mean number of fledglings/successful pair 2,5 (SD=0.8, N=32; Appendix 4). Sex ratio in 6 nests was 4 males and 9 females. Hobbies have seriously declined in The Netherlands during the last decade, probably by 50% or more. Although many factors may be involved, it is thought that Goshawk predation (particularly of nestlings, but also of adult birds) is the main reason. In many formerly well-populated areas, especially in woodland, Hobbies have almost disappeared as breeding birds. Kestrel F. tinnunculus: large variation in mean onset of laying between provinces (Table 11), on average 22 April (SD=12.7, N=186; Appendix 2) and between years (Appendix 8). Mean clutch size was 5.3 (SD=1.0, N=233; Appendix 3), mean number of fledglings/successful nest 4.6 (SD=1.2, N=342; Appendix 4). In 48 nests, all nestlings were sexed during ringing: 88 males and 131 females. Peregrine Falcon F. peregrinus: in 1996 two successful breeding attempts in nestboxes on power plants, with 3 and 2 fledglings. The female in one of the pairs was bora in 1988 and occupied the site since 1992.