Nest defense and egg recognition in the grey-backed thrush (Turdus hortulorum): defense against interspecific or conspecific brood parasitism?

Cuckoos (Cuculus spp.) parasitize the nest of many host birds; however, it is not clear why some potential hosts that occupy the same area as cuckoos are rarely or not used as hosts. Potential hosts might adopt a variety of strategies to defend against cuckoo parasitism, such as attacking adult cuckoos to keep them away from the nest, rejecting cuckoo eggs, or recognizing and refusing to feed cuckoo nestlings. We investigated the strategies used by grey-backed thrushes (Turdus hortulorum), a potential host of cuckoos, to prevent cuckoo or conspecific brood parasitism. Our results indicated that the attack rate of grey-backed thrushes on cuckoo dummies was very low (11.4%). Thrushes showed egg polymorphism and exhibited 100% rejection of non-artificial blue model eggs. Thrushes also failed to identify the eggs (8.3% of rejection) of conspecifics. Thus, grey-backed thrushes show a high egg rejection towards nonmimetic eggs, but low towards conspecific eggs. We suggest that egg recognition and rejection in grey-backed thrushes may be a strategy to cope with interspecific brood parasitism rather than conspecific brood parasitism, and have probably evolved due to previous cuckoo parasitism.