Data for: 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 host nest unnoticed. Using video recordings of 70
nests we showed that spotting a cuckoo at the nest did not increase
rejection rate of parasitic eggs, even if hosts saw the cuckoo repeatedly.
Hosts instead used the level of mimicry in background colour 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 the brood parasite at the
nest may not be as important as previously thought.