LEARNING TO RECOGNIZE NESTLINGS IS MALADAPTIVE FOR CUCKOO CUCULUS-CANORUS HOSTS

THE picture of a tiny passerine host feeding a huge cuckoo nestling challenges evolutionary biologists who explain animal behaviour as adaptive1-4. Cuckoo eggs sometimes resemble the eggs of the host, but nestlings of the common cuckoo, Cuculus canorus, look very different from the young of the host. The inability of the host to discriminate against such divergent nestlings is especially puzzling as some cuckoo hosts show a finely tuned discrimination ability between eggs5-8. Here I present a simple model to explain this paradox. The model shows that although learning to recognize eggs is adaptive, learning to recognize nestlings might not be. The mechanism of learned recognition, previously shown to maintain egg recognition, is unlikely to be adaptive for hosts like those of the common cuckoo, in which only the parasitic nestling remains in the nest. The reason that discrimination against parasite nestlings is not adaptive is that the cost of misimprinting (learning to recognize the parasite nestling as the parents' own) exceeds the benefit of correct learning. The model also explains why nestling discrimination is mostly found in host-parasite systems in which the parasite and the hosts' young are reared together1.