GREAT SPOTTED CUCKOOS IMPROVE THEIR REPRODUCTIVE SUCCESS BY DAMAGING MAGPIE HOST EGGS

Adult great spotted cuckoos, Clamator glandarius, damage the eggs of t heir magpie, Pica pica, host without removing or eating them. The numb er of damaged magpie eggs was recorded in 360 parasitized nests of whi ch 62.2% contained between one and eight damaged magpie eggs. Egg-dest roying behaviour may be adaptive if it reduces nestling competition an d/or enhances the hatching success of the cuckoo. To clarify the role of egg destruction for the reproductive success of great spotted cucko os, unparasitized magpie nests were experimentally parasitized (withou t egg damage) by introducing cuckoo eggs or chicks. Egg damage was com mon in parasitized nests but the eggs were not damaged by the hosts. E gg damage increased the breeding success of the cuckoos, by both reduc ing the number of competing host chicks in the nest and increasing the likelihood that late-laid cuckoo eggs would hatch. (C) 1997 The Assoc iation for the Study of Animal Behaviour.