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.