Egg Retrieval as a Cognitive Indicator in Cuckoo Hosts.

Egg retrieval behavior in hosts within avian brood parasitism systems was found to be regulated by the motivation to reject parasitic eggs. However, due to the limitations in the research systems, there is a lack of effective validation regarding the adaptation mechanisms of cuckoo hosts to the conflict between retrieving their own eggs outside the nest and rejecting parasitic eggs. This study uses Daurian redstarts (), a secondary cavity-nesting host parasitized by common cuckoos (), to verify the adaptive decision-making of the host between egg retrieval and egg rejection by simulating the occurrence of eggs outside the nest. The results showed that Daurian redstarts ignored 60.6% of highly mimetic conspecific eggs, with a retrieval rate of only 18.2%. Additionally, Daurian redstarts rejected 21.2% of conspecific eggs. However, non-mimetic budgerigar () and white model eggs were more likely to be directly rejected (75% and 86.4%, respectively) with no retrieval events. Our findings suggest that egg retrieval behavior in Daurian redstarts is likely influenced by the cognitive process of rejecting parasitic eggs, leading to occasional over-identification and difficulty in decision-making between egg retrieval and egg rejection, especially in the context of conflicting motivations.