Data from: True recognition of nestlings by hosts selects for mimetic cuckoo chicks

Cuckoos lay their eggs in other birds’ nests, whereafter the young cuckoo hatches, ejects its nestmates, and monopolizes the care of the host parents. Theory predicts that hosts should not evolve to recognize and reject cuckoo chicks because of the risk of mistakenly imprinting on a cuckoo chick in their first brood and thereafter always rejecting their own chicks. However, recent studies have revealed some hosts do reject cuckoo chicks from the nest, indicating that these hosts’ recognition systems either do not rely on first brood imprinting, or use cues that are independent of chick phenotype. Here, we investigate the proximate mechanisms of chick rejection behaviour in the large-billed gerygone (Gerygone magnirostris), a host of the little bronze-cuckoo (Chalcites minutillus). We find that gerygones use true template-based recognition based on at least one visual chick trait (the amount of hatchling down-feathers), and that this is further mediated by experience of adult cuckoos at the nest during egg-laying. Given the theoretical constraints of acquiring recognition templates via imprinting, gerygones must possess a template of own-chick appearance that is largely innate. This true recognition has facilitated the evolution of very rapid hatchling rejection and, in turn, striking visual mimicry of host young by little-bronze cuckoo chicks.

杜鹃（Cuckoo）会将卵产于其他鸟类的巢中，待杜鹃幼鸟破壳后，其会驱逐同巢的其他幼鸟，独占宿主亲鸟的抚育照料。理论预测，宿主不应演化出识别并拒斥杜鹃幼鸟的能力，因为存在如下风险：在第一窝雏鸟抚育中误将杜鹃幼鸟作为印记（imprinting）对象，此后便会持续拒斥自身的雏鸟。然而，近期研究表明部分宿主确实会拒斥巢中的杜鹃幼鸟，这意味着这些宿主的识别系统要么不依赖第一窝印记，要么使用不受幼鸟表型影响的识别线索。本研究以小青铜杜鹃（Chalcites minutillus）的宿主——巨嘴噪刺莺（Gerygone magnirostris）为研究对象，探究其拒斥幼鸟行为的近因机制。研究发现，巨嘴噪刺莺基于至少一种视觉幼鸟特征（雏鸟绒羽覆盖量）采用模板式识别（template-based recognition），且这一识别过程还会受到产卵期巢边成年杜鹃出现经历的介导。鉴于通过印记获取识别模板的理论限制，巨嘴噪刺莺必然拥有一套先天为主的自身雏鸟外观识别模板。这种真正的识别能力推动了极快速的雏鸟拒斥行为的演化，进而促使小青铜杜鹃幼鸟演化出针对宿主幼鸟的醒目视觉拟态。