How the grey bushchat combats parasitic eggs at different laying stages

This study was conducted from March to August between 2021 and 2023 in the Liuzhi area (26°13′ N, 106°42′ E), Guizhou, southwest China. The grey bushchat prefers to nest in cultivated fields, barren slopes, and under the roots of grasses or rocky cover in scrub forests, choosing relatively concealed nest locations. Parasitic cuckoo eggs are highly similar to host eggs in background color but are larger in size than host eggs. Although this population experiences a relatively low parasitism rate (8.2%, N = 1,116), it is the primary host utilized by the common cuckoo in this study area (Zhong et al., 2023).We systematically searched the area for breeding nests early in the breeding season and recorded their reproductive status, including stages such as nest building, egg laying, egg incubation, and brood rearing. Particular attention was given to the timing of the first egg laying, as this study aimed to determine whether the egg-laying event (whether the host has commenced egg-laying) influences egg recognition. For each nest, we recorded or measured basic reproductive parameters, including the date of the first egg, the number of eggs in the clutch, and the period of incubation. A GPS kit(web.gpstool.com) was used for site marking, and a camera (Canon EOS R50 + RF-S 18-45mm Kit) was used to photograph nests and eggs.Here, we explored the strategies of egg recognition and rejection of the grey bushchat (Saxicola ferreus haringtoni), 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 their own eggs in the nest. Furthermore, the host’s rejection behavior 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 egg-recognition mechanisms.