Intraspecific nest parasitism in the spotless starling Sturnus unicolor

Intraspecific nest parasitism in two colonies of Spotless Starling Sturnus unicolor breeding in nestboxes was studied in central Spain from 1991 to 19 94. Nests were monitored regularly and three criteria were used to detect n est parasitism: the appearance of more than one egg per day during the layi ng period of the host; the appearance of an egg after the start of incubati on; eggs with unusual shape or pigmentation. The proportion of parasitized nests in first clutches (37%) was twice that of intermediate (19%) or secon d (20%) clutches in colony B, whereas parasitism occurred in first (35%) an d intermediate (12%) but not in second clutches in colony A. Most clutches (52-70%) were parasitized during the host's laying period and received one parasitic egg. In 10% of the parasitized clutches in colony B, one of the h ost's eggs disappeared on the day the parasitic egg was added, suggesting t hat the parasitic female removed this egg. Although parasitism increased cl utch size significantly, it led to a decrease in host breeding success, mai nly through the removal of eggs and the loss of host nestlings and the surv ival of parasitic chicks. Observations suggested that parasitic females wer e young individuals without their own nests and/or those whose breeding att empt had been disrupted while laying in their own nest.