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Résumé :
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After twenty years of research on tree-breeding Swifts in the Harz Mountains this paper summarises some main results. Swifts are inhabiting natural deciduous forests on hillsides along the Bode and Selke rivers. Holes of Great and Middle Spotted Woodpeckers Picoides major, P. medius in living oaks Quercus spec. are preferred. An essential requirement for the full suitability of these holes are special processes of enlargement by rotting, which are shortly described. Holes in beeches Fagus sylvatica are less suitable. Data on abundance, size and development of the Swift population in the Harz Mountains are presented. The origin of the last remaining 'swift forests' in Germany are discussed by the knowledge of several other tree-breeding populations (e.g. in Colbitz-Letzlinger Heide, Mittelelbe, Sachsische Schweiz), and the current literature on forest development as well. These forests are mainly result of medieval forestry (former coppice, middle and pasture forest) and currently out of forestry use. Typical for all sites is the high age of the tree stock. This is the pre-condition for the development of an adequate number of wide-bodied holes. An exception is the Spessart forest (in Hesse), where the oaks can reach the necessary dimension because they are left until a high age before cutting. Tree-breeding Swifts are especially good indicators for historical old forests with very old trees. In Central Europe, their populations are virtually threatened by the. decline (through forestry and/or natural processes) of suitable old trees rich of large holes.
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