ADAPTATIONS OF BRAMBLINGS AND CHAFFINCHES TOWARDS PARASITISM BY THE COMMON CUCKOO

A coevolutionary arms-race between the common cuckoo, Cuculus canorus, and its hosts includes adaptations such as mimetic cuckoo eggs and counter-adaptations such as rejection of the parasitic eggs and aggression towards the parasite by the host. To investigate the existence of defence mechanisms in two species not so far reported as being cuckoo hosts in Norway, but considered to be suitable as hosts, experiments were carried out on bramblings, Fringilla montifringilla, and chaffinches, F. coelebs. Since there were no statistically significant differences between the two species in the way they treated foreign eggs, the data were pooled. The birds rejected about 80% of both non-mimetic and mimetic, artificial, cuckoo eggs, and many even ejected some conspecific eggs. Reports of cuckoo eggs mimicking brambling eggs, in Finland early in the present century, suggest a rapid spread of rejecter alleles. Although never observed in either bramblings or chaffinches, intraspecific nest parasitism is proposed as a possible factor to account for the rejection of conspecific eggs. The parent bramblings attacked a stuffed dummy cuckoo aggressively, but the presence of a stuffed cuckoo led to no increase in the rejection rate of conspecific eggs. © 1992 The Association for the Study of Animal Behaviour.