Reactions of red-backed shrikes Lanius collurio to artificial cuckoo Cuculus canorus eggs

To test the evolutionary arms race hypothesis (Dawkins and Krebs 1979) on the Red-backed Shrike-Cuckoo host-parasite relationship, a field test was carried out by placing artificial Cuckoo Cuculus canorus eggs into 52 Red-backed Shrike Lanius collurio clutches in Hungary. Two types of plastic eggs were introduced into shrike nests: (1) mimetic spotted eggs, and (2) blue eggs. Both types of eggs were placed into the nests either during the egg-laying period or at the beginning of the incubation period. The hosts ejected the foreign egg (71.2%), deserted the nest (19.2%), or accepted the foreign egg (9.6%). The blue egg was ejected more frequently than the mimetic egg. Desertion of nests in which the spotted egg was introduced was most frequent during the laying period, but after incubation started, females usually responded to the foreign egg by ejecting it. When ejection and desertion were treated together as rejection of the parasitic egg, no significant differences were found between the egg types and nest stages. Our results support the idea that Red-backed Shrikes are able to recognize their own eggs and reject parasitic eggs. This species could have been a host of the Cuckoo in the past, but when it learned to identify the parasitic eggs, the Cuckoo switched host species rather than evolving perfectly mimetic eggs to counteract the host's recognition ability.