Pinxten R., Eens M. & Verheyen R.F. (1991) Conspecific nest parasitism in the European Starling. ARDEA 79 (1): 15-30

From 1983 to 1988 we monitored 260 European Starling Sturnus vulgaris nests in three nestbox colonies around Antwerp (Belgium), for evidence for conspecific nest parasitism. Altogether 15% of 174 first clutches and 2% of 86 intermediate clutches were parasitized. The yearly proportion of first clutches with parasitic eggs varied from 0% to 37%. In most years, parasitism rate among first clutches was relatively high, despite an excess of available unoccupied nestboxes. Furthermore, parasitism rate was not related to nestbox occupancy indicating that nest parasitism in Starlings also evolved for other reasons than a storage of (suitable) nestboxes. Most parasitized first clutches were parasitized during the host's own laying period and received only a single parasitic egg. In at least 27% of the parasitized first clutches, one of the host's eggs disappeared on the same day as a parasitic egg was added, suggesting that the parasitic female was responsible for this egg removal. Parasitized first clutches, although having a significantly larger clutch size than normal first clutches, fledged significantly fewer young. Finally, the possible identity of parasitic females and possible anti-parasite tactics are discussed.