Deficiency in egg rejection in a host species as a response to the absence of brood parasitism

Different populations of a host species subject to variable patterns of selection due to cuckoo parasitism provide an optimal situation for studying natural selection and coevolution in action. We compared egg appearance and egg-rejection behavior of 2 common cuckoo (Cuculus canorus) hosts, the ashy-throated parrotbill (Paradoxornis alphonsianus) and the vinous-throated parrotbill (Paradoxornis webbianus) between mainland China and Taiwan population that have been segregated for 2–3 million years. Avian visual modeling showed that the mainland host population under strong selection from brood parasitism has evolved polymorphic eggs, while the island host population released from brood parasitism has maintained the original monomorphic egg phenotype. Furthermore, experiments indicated that under such long historical segregation, egg rejection in the island population decayed dramatically in the absence of cuckoo parasitism. This study provides strong evidence that egg-rejection ability can be dramatically deficient in host populations without brood parasitism compared to parasitized ones. The results further enhance our understanding of changes in egg-rejection behavior in birds without the selection pressure of brood parasitism for an extended period of time