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Résumé :
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The results of a twenty year (1979-1998) survey of a Sand Martin population in Middle Westphalia are presented. The study area covers 320 km2 along the river Lippe between Lippstadt (51degree42', 08degree24) and Paderborn (51degree44', 08degree41'), including about 50 gravel pits. In spite of a very high fluctuation rate there was a significant increase from 3 colonies with 225 burrows in 1979 to 11 colonies with 868 burrows in 1998. 1982 and 1984-1986 were years with very low population levels, whereas 1992 und 1997-1998 were peak years. Mean colony size was 75,6 burrows, 58,8% of all burrows were located in large colonies (> 100 burrows). The positive trend was not so much a result of an enlargement of existing colonies, but was caused by the colonization of new gravel pits. The population trend in our area was opposite to that in the whole of Northrhine-Westphalia, where a Sand Martins survey in 1998 revealed a decrease compared to the middle of the 1980s. The increase along the river Lippe is probably a regional phenomenon resulting from improved availability of nesting sites through intensive gravel excavation. As such artificial breeding sites offer no long-term perspective for breeding Sand Martins, natural river dynamics would have to be reestablished to secure the population. As long as Sand Martins depend on gravel pits and other anthropogenic breeding sites, the survival of the population in Northrhine-Westphalia must be regarded as threatened. The species should therefore be maintained in the respective Red Data Book category.
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