KONING FJ (1986) Short contributions: Take-overs by Jackdaws Corvus monedula ofnest sites occupied by Tawny Owls Strix aluco. LIMOSA 59 (2): 91-91.

Western Jackdaw Coloeus monedula Tawny Owl Strix aluco From 1981 onwards, several cases of a successful takeover by Jackdaws of nest sites occupied by Tawny Owls in the Amsterdamse Waterleidingduinen (NH) were noticed. The Jackdaws simply dumped large numbers of twigs into the nestboxes on top of the breeding Tawny Owl, in one case even amounting toa layer with a thickness of 25 cm. This strategy forces the Tawny Owl to desert the nestbox, and once a breeding Tawny Owl with two young of c. two weeks old were unable to escape and found dead in the nestbox under a successful nest of Jackdaws. In the area of 3300 ha, Jackdaws increased, whereas Tawny Owls decreased, though not necessarily as a result of nest site competition by Jackdaws (tab. 1).

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