How the grey bushchat combats parasitic eggs at different laying stages

Under the strong selective pressure of parasitism, hosts have evolved the strategy to recognize and reject parasitic eggs or chicks. Although studies have shown that hosts use multiple egg recognition mechanisms, it remains unclear whether they can flexibly deploy specific strategies across different parasitism contexts. Here, we explored the strategies of egg recognition and rejection of the grey bushchat (Saxicola ferreus), a host of the common cuckoo (Cuculus canorus), across different stages of egg-laying. The results indicated that the grey bushchat employed an "onset of laying" mechanism to recognize foreign eggs during the pre-laying stage, with significantly higher rejection rates in this period than post-laying. Additionally, the grey bushchat could remember the phenotype of its own eggs through a short learning process during the first egg-laying episode, without requiring physical presence of its own eggs in the nest. Furthermore, the host's rejection behaviour was shaped by the degree of egg mimicry and trade-offs associated with nest desertion costs. Our study reveals that the grey bushchat flexibly adopts distinct egg recognition and rejection strategies in response to varying parasitic contexts. Future research should incorporate individual variation in host reproductive background to better understand the evolution of plastic egg-recognition mechanisms.