Do common cuckoos’ vocalisations affect egg discrimination in the strong rejector red-backed shrike?

Abstract Avian brood parasitism is a widespread reproductive strategy in which parasitic birds exploit host nests to raise their offspring, often at a significant cost to the host species. Hosts counter parasitism through various defensive mechanisms, including egg rejection, which the either the visual presence or the vocalisations of the parasite near the nest can trigger. This study aimed to assess the response of Red-backed Shrikes ( Lanius collurio ), a species formerly commonly parasitised by the Common Cuckoo ( Cuculus canorus ) to simulated brood parasitism based solely on auditory cues, using playbacks of cuckoo vocalisations. Over two breeding seasons in eastern Poland, artificial Cuckoo eggs were placed in 63 Shrike nests, followed by playback experiments. The results revealed no significant effect of simulated parasite presence on egg rejection rates, and the playback of the Sparrowhawk call also did not affect the host’s responses. However, the observed pattern suggests a possible influence of incubation advancement on host responses. Defensive behaviour peaked during the egg-laying and early incubation phases, with over 90% of eggs rejected, and fell slightly during advanced egg incubation. These findings suggest that Shrikes do not rely on auditory cues to detect parasitic threats. This study highlights the critical role of the breeding stage in shaping anti-parasitic strategies, and suggests that a strong ability to recognise parasitic eggs may reduce dependence on additional sensory cues. Further research is needed to examine the interplay between auditory and visual stimuli across different host species and geographic regions.