Recognition and rejection of parasitic eggs are crucial host defenses against brood parasitism. Template-based recognition and recognition by discordancy are two main mechanisms of egg recognition used by bird hosts. However, even in hosts that utilize template-based recognition, a decrease or absence of templates in the nest may affect parental judgment when the disparity between foreign and host eggs is small. This study examined two sympatrically breeding bunting species, the Godlewski’s Bunting ( Emberiza godlewskii ) and Yellow-throated Bunting ( E. elegans ), both of which are potential hosts of the Common Cuckoo ( Cuculus canorus ). To examine the egg recognition mechanisms, we introduced red model eggs, Budgerigar ( Melopsittacus undulatus ) eggs, inter-specific eggs, and conspecific eggs at different ratios of host to foreign eggs and observed the rejection behaviors by two bunting species. The results showed that both bunting hosts mainly employed template-based recognition to reject foreign eggs. However, when the disparity between the foreign and host eggs was minimal, both species exhibited a decrease in rejection rates as the number and ratio of host eggs decreased. Our findings support the perspective that repeated egg-laying by cuckoos may contribute to enhancing the success rate of parasitism. However, constrained by the density of cuckoo populations and the trade-off between reproductive investment and returns for cuckoos, the two bunting species with strong egg recognition abilities and template-based recognition are rarely parasitized.
Zhang et al. (Fri,) studied this question.