Egg-Inspection Frequency Predicts Egg Recognition but Not Ejection in a Cuckoo Host

One of the most effective strategies for avian hosts to defend against brood parasitism is to reject the parasitic egg from the focal nest. However, substantial inter-individual variability in egg-rejection behavior remains unexplained. A critical prerequisite for egg ejection is the recognition of the parasitic egg. Hosts may improve recognition accuracy by increasing inspection frequency and duration, thereby facilitating comparisons between the foreign egg and their own eggs. In this study, we investigated the relationship between egg inspection and egg-rejection behavior in the Daurian redstarts Phoenicurus auroreus, a common host of the common cuckoo Cuculus canorus. Following the introduction of a model cuckoo egg, redstart females increased both the frequency and duration of egg inspection. Moreover, individuals that inspected the parasitic egg more frequently and for longer were more likely to recognize it. However, inspection behavior was not associated with subsequent egg ejection. Therefore, our results demonstrate that egg inspection enhances recognition of the parasitic egg but does not directly translate into ejection decisions.