Havik Accipiter gentilis legt superdwergei, of: leven en dood in een 30-jarig territorium op het voedselarme Planken Wambuis (Veluwe)

The average Northern Goshawk egg in The Netherlands measures 56.9 x 43.9 mm (volume 56.1 cc), based on a large sample across regions and years (n= 1011; Bijlsma 2003). In 2003, a 12-year old female Goshawk produced a single runt egg, measuring 31.5 x 23.1 mm (8.57 cc), by far the smallest recorded so far in The Netherlands. The year before, this female had been incubating for some time on an empty nest. These findings sparked off a reconstruction of the life history of this female, embedded in the history of this particular site. From 1975 onwards, this territory has been successively occupied by six female Goshawks (Table 1, Appendix 1). It is situated on Planken Wambuis, a study area of 1965 ha in the Veluwe district of the central Netherlands (52°03’N, 5°48’E), where 3701 field hours were spent doing fieldwork on 759 days in 1968-May 2003. The region mostly consists of coniferous forests (Scots pine Pinus sylvestris), heaths (Calluna vulgaris) and fields on coarse sandy soil, representing one of the poorest habitat types in The Netherlands. The local Goshawk population increased in the early 1970s, reached a peak of 6-7 pairs in 1976-86, then declined to 3-5 pairs in the 1990s and early 2000s (Figure 1). Total fledgling production showed a steep and consistent decline from 14-18/year in 1975- 81 to 4-6/year in 1997-2002 (Figure 1). The initial increase was most likely a recovery from pesticidecaused losses in the 1960s and early 1970s. The decline since the late 1980s was mainly caused by a steep reduction in avian biomass, starting in the late 1970s, continuing into the 1990s, and levelling off in the early 2000s. The life history of a single territory located in the southern part of Planken Wambuis was reconstructed on the basis of systematic fieldwork, i.e. collection of moulted feathers (thus being able to identify individual females), location and use of nesting trees, determination of clutch size, egg size, breeding success and condition of nestlings, and systematic sampling of prey remains. The territory became occupied for the first time in 1975. In 1975-2003, twelve different nesting trees were used, viz. 3 Scots pines Pinus sylvestris (7 years), 5 hybrid Larches Larix leptolepis (9 years), 3 Beeches Fagus sylvatica (8 years) and 1 Douglas fir Pseudotsuga menziesii (5 years in a row), covering an area of 1400 x 900 m (Figure 2). None of the nesting trees, so far, has been in use for more than five years, the normal procedure being alternating usage of existing nests and building new ones every now and then (Appendix 1): 10x once, 4x twice, 2x thrice and 1x at least five times. Nest switching seemed independent of breeding success in the preceding year. Since 1975, six different females consecutively occupied this site, their presence varying in duration from l-> 10 years (Table 1). Take-overs were accompanied by the disappearance of the previous female, presumably signifying the death of the latter (comparison of moulted feathers in this and neighbouring territories always failed to track down the disappeared female). Change-overs were sometimes (females C and D; the C-bird was a falconer’s escapee), but not always (females B and E), preceded by poor breeding performances (Table 1). The longest (and still) living female F started breeding in 1994 in her third year of life, showed a peak- performance in her 4th through 6th year of life (3x 4 eggs, 10 fledglings), then started to produce smaller clutches (3x 3, 1x 4 eggs) with slightly declining egg volume (Figure 3) and increasing losses during the various nest stages (1x3 eggs 2x 1 nestling). In her 11th year of life, she incubated on an empty nest, and in her 12th year of life she produced an extremely small runt egg (see above, Photo). This decline in reproductive performance is thought to have been age-dependent, although information on the male(s) is insufficient.