De Havik Accipiter gentilis op de Utrechtse Heuvelrug van 1965-70: broedresultaten, prooiresten en ruiveren

This study describes the fortunes of a Northern Goshawk population in the central Netherlands during the late 1960s (c. 150 km², mostly forested, situated between the towns of Hilversum and Veenendaal). In this period, the Dutch Goshawk population was in steep decline from a relative peak in numbers in the 1950s; its nadir was reached around 1968-69, resulting from widespread use of persistent pesticides in agriculture. The study area was systematically searched for Goshawk nests. Annually, each territory was visited frequently to check for nest occupation and changes in nest site, breeding success, number of fledglings (observations from the ground), moulted feathers (to age and individually recognise breeding birds) and prey remains (packings, cf. Appendix 1 and 2). During the six years of the present study six territories were located, all nestings taking place in Scots pine Pinus sylvetris (13x) or beech Fagus sylvatica (8x). The occupation rate of territories varied between 1 and 6 years each and the number of fledglings between 0.0 and 2.8 per breeding attempt (Appendix 1). Taken together, the six territories were occupied for 21 territory-years (occupation rate of 67%) and fledged 29 young (on average 1.38 fledgling/breeding attempt, or 2.42 fledglings per successful breeding attempt). Based on findings of moulted feathers (Appendix 4) and field observations, the age of nesting birds could be established in most instances. In three cases, one member of the breeding pair was in first-year plumage, i.e. the female of Territory I in 1968 (paired to adult male; the latter had been present in 1966 as well), the male in Territory IV in 1970 (paired to adult female), and the male in Territory V in 1967 (paired to adult female). In three breeding seasons from which detailed information was available (1966-68, with resp. 4, 4 and 3 territories), at least eight changes in pair composition were recorded. As compared to the 1970s and 1980s, when the Dutch Northern Goshawk population successfully recovered from the decline in the 1960s, this study in the late 1960s shows many signs of pesticide-related stress and – possibly – persecution, as for example in the frequency of turnover (on average each pair member was replaced after two years; Appendix 4), some territories apparently being occupied by a solitary female (Territory II in 1968, Territory IV in 1966-69; indicating lack of non-breeding floaters?), instances of clutches apparently failing to hatch (Territory I in 1967; Territory III in 1967,1968 – when egg remains were found underneath the nest – and 1970), poor nesting success (9 out of 21 nests failing to raise fledglings, i.e. 43%), low density (3- 6 pairs on c. 150 km² of mostly woodland), contamination of feathers with Hg and fast turnover of breeding pair members in Territory 5 (where racing pigeon was the most common prey species, and pigeon fanciers abounded in the vicinity; this may have resulted in human persecution). It is unlikely that food shortage played a role in these events. The prey list for the breeding season (n=368) shows 24 avian species and 2 mammalian species (Appendix 2- 3), of which only four prey species were responsible for 66.6% of all prey, i.e. racing pigeon Columba livia (22.3%), woodpigeon C. palumbus (17.1%), rabbit Oryctolagus cuniculus (16.3%) and jay Garrulus glandarius (10.9%). This dominance of few prey species in the weight category of 100-500 g can be regarded as an indication of high availability of profitable prey. Therefore, the above-mentioned examples of poor reproductive success and – apparently – high mortality can best be explained by the side-effects of massive application of persistent pesticides in farmland in the 1960s, possibly in combination with local persecution. Following stepwise bans of Dieldrin, Aldrin, Heptachlor. mercury and DDT between January 1968 and 1 July 1973, the Goshawks on the Utrechtse Heuvelrug started to recover quickly. An indication of recovery was noted in 1970, when a new territory (Ilia, already once used as an alternate breeding site for Territory III in 1968) became occupied by new pair. In a part of the study area, where 3 territories were recorded in 1970, already 13 territories were located in 1977-78 (of which 12 were successful, including a pair in which both pair members were in first-year plumage; P. Opdam). This trend has continued. In the Estate of Den Treek (Territory II in 1965-67, not occupied in 1968-70), for example, five nesting pairs were present in 2001 (data: G. van Haaff & H. Schuurmans). A similar trend was noticed for Common Buzzard Buteo buteo (first breeding in 1968, nowadays by far the commonest raptor in the region) and Eurasian Sparrowhawk Accipter nisus (failed nest in 1968, first successful breeding in 1975). Adres: Reijnier Vinkeleskade 25, 1 hoog, 1071 SR Amsterdam (t.lent@chello.nl)