Rejection of common cuckoo Cuculus canorus eggs in relation to female age in the bluethroat Luscinia svecica

The evolutionary equilibrium hypothesis explains the existence of both acceptors and rejecters of brood parasite eggs within a host population as resulting from a balance between the costs of acceptance and the costs of recognition errors. In such equilibria conditional responses may play an important role. One such response that has been demonstrated in one common cuckoo Clicidus canorus host species is that first year, naive breeders accept parasitic eggs at a higher frequency than older and more experienced birds do. In the present study we tested whether this is the response in the bluethroat Luscinia svecica. We did not find any difference in rejection behaviour between first-year breeders and older birds. This finding is discussed in relation to recognition costs, cuckoo egg mimicry and the bluethroat's present status as a host. We conclude that the results are best explained by the evolutionary lag hypothesis.