The mere presence of cuckoos in breeding area alters egg-ejection decisions in Daurian redstarts

Brood parasitic cuckoos and their hosts serve as model systems for studying host-parasite coevolution. Egg-rejection behavior constitutes an effective defense against brood parasitism, but some host species show phenotypic plasticity in egg-rejection behavior. Direct exposure to a cuckoo near the nest can increase egg-rejection likelihood, and long-term studies have shown that increased the egg-rejection rates generally correlate with higher parasite prevalence. However, it remains unclear whether such increases result from interactions between parasites and hosts, as these can be surprisingly common, or whether the mere presence of cuckoos in the breeding area is sufficient. Daurian redstarts Phoenicurus auroreus are a common host of the common cuckoo Cuculus canorus that defend against cuckoo parasitism mainly by ejecting the parasitic egg from the nest. This species is unique, as its first breeding attempt of the year takes place before the arrival of cuckoos, excluding the possibility for direct interactions. We simulated the ambient presence of cuckoos or hoopoes Upupa epops (control) in sub-populations of redstarts during their first egg-laying period by presenting taxidermic models and playing back vocalizations. Redstarts in cuckoo-treated plots showed significantly higher egg-ejection rates than individuals in control plots, even though females in both groups were equally likely to recognize the parasitic egg. Among females that did recognize the parasitic egg, those exposed to the cuckoo treatment were more likely to eject it than those exposed to the control treatment. Our results demonstrate unequivocally that the mere presence of cuckoos in the environment is sufficient to provoke egg-ejection behavior.

育雏寄生杜鹃（Brood parasitic cuckoos）与其宿主是探究宿主-寄生虫协同进化的经典模式系统。弃卵行为是对抗育雏寄生的有效防御策略，但部分宿主物种的弃卵行为存在表型可塑性。直接接触巢区附近的杜鹃可提升宿主的弃卵概率，而长期研究表明，弃卵率的提升通常与寄生虫侵染率的升高呈正相关。然而，目前仍不清楚此类弃卵率的提升究竟源于寄生虫与宿主间的直接互作（此类互作实则相当普遍），还是仅需繁殖区域内存在杜鹃即可触发该行为。北红尾鸲（Daurian redstart，*Phoenicurus auroreus*）是大杜鹃（Common cuckoo，*Cuculus canorus*）的常见宿主，其对抗杜鹃寄生的主要策略为将寄生卵逐出巢外。该物种的独特之处在于，其年度首次繁殖活动会在杜鹃抵达前启动，因此完全排除了宿主与杜鹃发生直接互作的可能。本研究在北红尾鸲的首次产卵期，通过放置剥制标本（taxidermic models）并播放鸣唱回放（playing back vocalizations）的方式，对其不同亚种群分别模拟了繁殖环境中杜鹃或戴胜（Upupa epops，作为对照）的存在。经杜鹃处理的样地内的北红尾鸲，其弃卵率显著高于对照样地个体，尽管两组雌性识别寄生卵的概率并无显著差异。在成功识别寄生卵的雌性个体中，经杜鹃处理组的个体比对照组个体更倾向于执行弃卵行为。本研究结果明确证实，仅需繁殖环境中存在杜鹃，即可触发北红尾鸲的弃卵行为。