JOURNAL OF AVIAN BIOLOGY 33: 366–370, 2002 Rejection of common cuckoo Cuculus canorus eggs in relation to female age in the bluethroat Luscinia secica Trond Amundsen, Paul T. Brobakken, Arne Moksnes and Eivin Røskaft Amundsen, T., Brobakken, P. T., Moksnes, A. and Røskaft, E. 2002. Rejection of common cuckoo Cuculus canorus eggs in relation to female age in the bluethroat Luscinia secica. – J. Avian Biol. 33: 366–370. 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 Cuculus 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 secica. 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. T. Amundsen, P. T. Brobakken, A. Moksnes (correspondence) and E. Røskaft, Depart - ment of Zoology, Norwegian Uniersity of Science and Technology, NTNU, Realfag - bygget, N-7491 Trondheim, Norway. E-mail: Arne.Moksnes@chembio.ntnu.no Interactions between avian brood parasites and their hosts have received much attention as model systems for coevolution in the last three decades (Rothstein 1990, Rothstein and Robinson 1998, Davies 1999, 2000). Interspecific brood parasites lay their eggs in the nests of other bird species and leave incubation and subsequent parental care to the foster parents. In the common cuckoo Cuculus canorus, which parasitises small passerines, different strains or gentes have evolved. These gentes have become specialised on dif- ferent host species whose eggs they typically mimic (Chance 1940, Baker 1942, Moksnes and Røskaft 1995). After hatching, the cuckoo chick evicts all the eggs or young of the host and is raised by the foster parents. The consequence of such parasitism is a selec- tion pressure on hosts for recognition and rejection of cuckoo eggs. Rejection of parasitic eggs that match those of the host poorly will in turn select for better egg mimicry in the cuckoo. These interactions are hypothe- sised to result in an arms race between host and para- site, and may ultimately lead to the evolution of specialised gentes, as described by Davies and Brooke (1989a). The arms race hypothesis has been experimen- tally supported by Davies and Brooke (1989b) and Moksnes et al. (1990). Even if it is beneficial for hosts to reject cuckoo eggs, in many host species a proportion of the individuals accepts parasitic eggs. In fact, there is a mixture of rejecters and acceptors within many species (see e.g. Rothstein 1990). The causes of such variation in rejec- tion behaviour have been much debated. Two main hypotheses explain why some species have intermediate rejection rates: (1) the evolutionary lag hypothesis (Rothstein 1975a, Dawkins and Krebs 1979, Davies and Brooke 1989a, Moksnes et al. 1990), and (2) the evolutionary equilibrium hypothesis (Zahavi 1979, Ro- hwer and Spaw 1988, Brooker and Brooker 1990, Petit 1991, Lotem et al. 1992, Lotem and Nakamura 1998). The evolutionary lag hypothesis states that it is al- ways adaptive to reject parasitic eggs, but such be- haviour may still not have appeared (no rejection) or spread to fixation in a host population (Rothstein 1975b). According to the evolutionary equilibrium hy- pothesis, the response towards the parasitic egg is deter- mined by a compromise between the costs of acceptance and the costs of recognition errors. © JOURNAL OF AVIAN BIOLOGY JOURNAL OF AVIAN BIOLOGY 33:4 (2002) 366