Egg management strategies of a common cuckoo host in response to variable egg phenotypes across three geographical populations

Egg retrieval refers to the behavioural process through which birds recover eggs that have inadvertently rolled out of their nests. In contrast, avian hosts of brood-parasitic cuckoos exhibit a counteradaptation termed egg rejection, by which they remove foreign eggs to optimize reproductive success. Both behaviours require cognitive egg recognition mechanisms, and variation in host rejection across populations probably reflects differences in egg phenotypic diversity. Recent studies suggest that egg rejection behaviour can modulate egg retrieval tendencies, as the fitness costs of mistakenly retrieving parasite eggs can reduce the likelihood of retrieval in parasitized populations. This study examined egg retrieval behaviour in the oriental magpie-robin, Copsychus saularis, a frequent host of the common cuckoo, Cuculus canorus. We presented either artificial blue or white eggs, as well as conspecific eggs, to hosts from three geographically distinct populations in China: Hainan, Guangxi and Guizhou. Results indicated that oriental magpie-robins preferentially retrieved conspecific eggs while systematically rejecting both blue and white model eggs. Notably, rejection rates for white eggs marginally exceeded those for blue ones. Geographical variation significantly influenced behavioural responses: conspecific eggs were more frequently retrieved in Guangxi, whereas ignoring was more common in Hainan and Guizhou. Model eggs elicited similarly high rates of ignoring and ejection across all populations, suggesting either enhanced egg discrimination capabilities or reduced parasitism pressure in the Guangxi populations. These findings underscore the critical roles of egg phenotype and geographical adaptation in shaping egg management strategies among avian hosts.

卵回收（egg retrieval）是指鸟类将不慎滚出巢外的卵取回巢内的行为过程。与之相对，育雏寄生杜鹃（brood-parasitic cuckoos）的鸟类宿主会演化出名为卵排斥（egg rejection）的反适应策略，通过移除外来卵以提升自身繁殖成功率。上述两种行为均依赖认知层面的卵识别机制，而不同种群宿主的卵排斥行为差异，大概率反映了卵表型多样性的区别。近期研究表明，卵排斥行为可调控卵回收倾向：因误取回寄生卵所产生的适合度成本，会降低被寄生种群的卵回收概率。
本研究以东方鹊鸲（oriental magpie-robin，*Copsychus saularis*）为对象，该物种是普通杜鹃（common cuckoo，*Cuculus canorus*）的常见宿主之一，旨在探究其卵回收行为。我们为来自中国海南、广西、贵州三个地理分隔种群的宿主，分别提供了人工蓝色卵、白色卵以及同种卵作为实验材料。实验结果显示，东方鹊鸲会优先取回同种卵，同时系统性地排斥蓝色与白色的模型卵。值得注意的是，白色模型卵的排斥率略高于蓝色模型卵。
地理差异对宿主的行为反应存在显著影响：广西种群对同种卵的回收频率更高，而海南与贵州种群则更倾向于忽略同种卵。所有种群对模型卵的忽略率与驱逐率均处于较高水平，这表明广西种群要么具备更强的卵识别能力，要么面临更低的寄生压力。本研究结果凸显了卵表型与地理适应在塑造鸟类宿主卵管理策略中的关键作用。