Potential cuckoo hosts have similar egg rejection rates to parasitized host species

Background Thrush species are rarely parasitized by cuckoos, but many have a strong egg recognition ability. To date, there is a limited understanding of the relationship between host egg rejection and cuckoo parasitism rate.

Methods By using egg experiments in the field, we compared egg rejection between two non-parasitized potential host species and two parasitized hosts of cuckoos in the same region.

Results The White-bellied Redstart (Luscinia phoenicuroides), a host of the Common Cuckoo (Cuculus canorus), rejected 66.6% of blue model eggs; the Elliot's Laughingthrush (Trochalopteron elliotii), a host of the Large Hawk Cuckoo (Hierococcyx sparverioides), rejected 25% of blue model eggs and 46.1% of white model eggs; and the Chestnut Thrush (Turdus rubrocanus) and the Chinese Thrush (T. mupinensis), in which cuckoo parasitism has not been recorded, rejected 41.1 and 83.3% of blue model eggs, respectively. There were no significant differences in the egg rejection among them, although the Chinese Thrush showed the highest rate of egg rejection.

Conclusions This study indicates that the egg recognition ability of cuckoo hosts has no correlation with the actual parasitism rate of cuckoos. We suggest that the egg recognition ability of the two potential host species may have been retained from a parasitic history with the cuckoo, while the two common host species have developed their egg rejection abilities due to current parasitism pressure. In addition, our study highlights the importance of the multi- cuckoo parasite system for better understanding the selection pressure of parasitism on the evolution of host egg recognition abilities.