Predatie van Vossen Vulpes vulpes in kolonies van Oeverzwaluwen Riparia riparia: over slimme streken en hoge sprongen

In 2000 I established predation by Red Foxes in some colonies of Sand Martins in a military exercise area in the centre of the Province of Drenthe. The nests were mainly predated from above, from where the foxes easily dug straight to the nesting chambers. The results were published and lead to great rumour among nature conservists. In reaction on this rumour the sand walls are scraped now before every breeding season to keep the walls steep, the height of the walls has been increased to 1,5-2m and the ground directly above the nesting walls is at many sites covered with tarpaulin. The protection lead to reduced predation, especially from above in the tarpaulin covered sites (Table 1, 2 and Appendix 1). Predation from the front side did not significantly increase nor was it reduced. Despite the increased height of the walls, the foxes intensified their attempts from a maximum height of 77 cm (from floorto nest entrance) to 136 cm. Most of these attempts however turned out to be not successful (Table 2). Only it was visible by scraping tracks that the foxes had reached the nest entrance. The predation rate in 2000 (35%), before measures were taken, did not result in a declining population in the next year. Apart from 2003 the population grew each year to a higher level. If seems that for number of breeding pairs in our colony, availability of suitable breeding sites was more important than predation level.