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

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