MOTIVES FOR PARENTAL INFANTICIDE IN WHITE STORKS CICONIA-CICONIA

White Storks Ciconia ciconia parents were observed to kill their smaller chick in 9 out of 63 nests observed during a three-year study. Infanticidal parents were caring for larger broods and laid larger clutches than non-infanticidal birds. Males killed the chick in 8 out of 9 cases. Victims were born from the last-hatched egg in 4- and 5-egg clutches, they were the lightest in their brood and grew at lower rates than their nestmates during the days preceding their elimination. The last-hatched nestlings in 4-chick broods had lower pre-fledging survival rates than their elder sibs. Potential victims contributed a low fraction to parents' reproductive output, and 4-chick broods were especially costly to raise because parents provisioned them both more frequently and for longer nestling periods. Hence, the presence of an extra chick seems to lower the benefit/cost ratio to parents rearing a large brood once its elder siblings have hatched successfully. If nestlings do not compete aggressively for food, parents would be selected to eliminate the extra chick themselves. This hypothesis could provide an explanation for the existence of parental infanticide also in other species.