Egg rejection does not change with seasonal variation in risk of parasitism in two sympatric breeding cuckoo hosts

This study was conducted from March to August 2020–2024 in the Liuzhi area of Guizhou, southwestern China (22°13′ N, 106°42′ E). The study area is located in a temperate monsoon climate zone with an elevation range of 1,070–1,657 m. The area is mainly a karst landscape consisting of a mosaic of villages, farmland, scrublands, and barren slopes (Zhong et al., 2023).This study focuses on two sympatric hosts of the common cuckoo (Cuculus canorus), the grey bushchat (Saxicola ferreus) and the black-throated stonechat (Saxicola torquata). Using experimental eggs with varying degrees of mimicry (representing different egg rejection rates), we tested whether these hosts adjusted their egg rejection intensity based on the current cuckoo parasitism risk. Our results showed that both host species exhibited notable differences in rejection rates among various model eggs. However, regardless of the model egg type, no significant differences in egg rejection rates before and after the arrival of the cuckoo was noted. This finding suggests that the presence of cues associated with the arrival of cuckoos does not trigger changes in the hosts’ egg rejection rates. Additionally, we found that egg rejection rates could either increase or decrease after the arrival of the cuckoo. Therefore, the hypothesis of egg rejection adjusting for seasonal changes in parasitism risk was not supported by the present study. Future studies should investigate various host life history traits, such as anti-parasitism adaptation patterns, breeding experiences associated with age structure and mating systems, to further explore the mechanisms underlying seasonal changes in host egg rejection.