Is egg-damaging behavior by great spotted cuckoos an accident or an adaptation?

Adult great spotted cuckoos Clamator glandarius damage the eggsof their magpie Pica pica host without removing them from thenest or eating them but by producing the death of the embryo.Observations as well as experiments were used to test severalpredictions of two different possibilities: great spotted cuckooegg-damaging behavior is a parasitic tactic resulting froma direct selection process (the adaptation hypothesis), oregg damage is caused by thick-shelled cuckoo eggs which evolvedto avoid breakage during rapid laying (the nonadaptation hypothesis).Previously, we provided experimental evidence that egg damageincreased the breeding success of cuckoos when they laid lateduring the laying sequence of the magpie. However, when theylaid early, egg-damaging behavior did not increase cuckoo breedingsuccess, contrary to the adaptation hypothesis. In an experimental study, when we simulated laying behavior by the great spottedcuckoo, we found that (1) the number of damaged magpie eggswas significantly lower than in natural parasitism, and (2)whereas in the experimental manipulations the number of damagedeggs did not depend on the number of magpie eggs, in natural parasitism, the number of damaged eggs increased with clutchsize of the magpie. These results support the predictions ofthe adaptation hypothesis, implying that egg damage is notan incidental consequence of rapid egg laying, but an adaptation.