Egg mimicry, not the sight of a common cuckoo, is the cue for parasitic egg rejection

Many studies have found that if hosts observe a brood parasite at their nest, they use it as a cue to reject parasitic eggs. However, most previous work has simulated brood parasitic events by exposing a stuffed parasite near a host’s nest. Responses to the presence of a real parasite have not yet been adequately studied under natural conditions. We therefore investigated whether great reed warblers (Acrocephalus arundinaceus) are more likely to reject a parasitic egg if they see a parasitizing common cuckoo (Cuculus canorus) at their nest than if the parasite approaches the host nest unnoticed. Using video recordings of 70 nests, we showed that spotting a cuckoo at the nest did not increase the rejection rate of parasitic eggs, even if hosts saw the cuckoo repeatedly. Hosts instead used the level of mimicry in the background color for cuckoo egg rejection. Since not every visit by the cuckoo leads to parasitism, seeing the brood parasite may not be a reliable enough cue for the host. Therefore, our results suggest that the sight of a cuckoo at the nest may not have as severe consequences for it as previously thought.