SPAANS AL & RENSSEN TA (1983) Effects of controlling Crows Corvus corone and Jackdaws C. monedula on numbers. LIMOSA 56 (2): 37-44.

From November 1981 to November 1982 we censused up to three times a week all Crows Corvus carone and Jackdaws C. monedula (the latter only from mid-July onwards) present in the Arnhem Zoo (45 ha) (fig. 1). During the same period 337 Crows and 223 Jackdaws were eliminated from the study area, mainly by displacement of the birds to Kampen, 61 km N of the zoo. Displacement appeared very effective for first-year (initial date 1 July) Crows and Jackdaws: only two Crows and no Jackdaws were retrapped in the zoo after displacement (expected numbers based on retraps of non-displaced birds (tab. 1,2) 97 and 38, respectively). Recoveries from outside the zoo showed that the birds either remained near Kampen or dispersed randomly, Crows up to 75 km of the place of releasing (fig. 2), Jackdaws up to 4 km. Crows older than one year, however, appeared to return to the zoo from the end of the winter onwards (four retraps, five expecte'd, earliest retrap 25 February). However, only a few Crows of this age group have been. displaced after 1 March. No Jackdaws older than one year have been displaced. Even during periods of high control pressure, the numbers of Crows and Jackdaws remained stable (tab. 3,4). Evidently, open places were rapidly occupied by birds from elsewhere. The immigrants carne mainly from nearby areas, as indicated by the capture of twelve birds which had been ringed within 5 km of the zoo. At the end of the study period the number of Crows was nearly twice as high as the year before. It is suggested that empty places were occupied predominantly by low-ranked wandering Crows expelled from groups elsewhere. Being low-ranked these immigrants are presumably less capable to keep out conspecifics than high-ranked birds resulting in the inclusion of more birds in the study area than before Western Jackdaw Coloeus monedula Carrion Crow Corvus corone

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