Data from: To call or not to call: parents assess the vulnerability of their young before warning them about predators

Communication about predators can reveal the effect of both conspecific and heterospecific audiences on signalling strategy, providing insight into signal function and animal cognition. In species that alarm call to their young, parents face a fundamental dilemma: calling can silence noisy offspring and so make them less likely to be overheard, but can also alert predators that young are nearby. Parents could resolve this dilemma by being sensitive to the current vulnerability of offspring, and calling only when young are most at risk. Testing whether offspring vulnerability affects parental strategy has proved difficult, however, because more vulnerable broods are often also more valuable. We tested experimentally whether parent white-browed scrubwren, Sericornis frontalis, assessed brood noisiness when alarm calling. When a model predator was nearby, parents gave more alarm calls when playbacks simulated noisy broods, yet brood noisiness did not affect adult calling when only a control model was present. Parents were therefore sensitive to the tradeoff between silencing young and alerting predators to the presence of nests. Our study demonstrates that receiver vulnerability can affect signalling decisions in species other than primates.针对捕食者的通讯行为可揭示同种（conspecific）与异种（heterospecific）信号接收者对信号传递策略的影响，为解析信号功能与动物认知提供关键洞见。对于会向幼崽发出警戒叫声的物种而言，亲代面临一项根本性两难困境：发出警戒叫声虽可让吵闹的幼崽安静下来，降低其被窃听的概率，但同时也可能会向捕食者暴露幼崽的藏身位置。然而，检验子代易受攻击性是否会影响亲代的信号策略一直颇具挑战，因为易受攻击性更高的窝雏通常也具备更高的繁殖价值。我们以白眉丝刺莺（Sericornis frontalis）为实验对象，探究亲代在发出警戒叫声时是否会评估窝雏的吵闹程度。当捕食者模型靠近时，若通过声音回放模拟出吵闹的窝雏情境，亲代会发出更多的警戒叫声；而当仅存在对照模型时，窝雏的吵闹程度并不会影响成鸟的叫声输出频次。由此可见，亲代能够权衡“沉默幼崽”与“向捕食者暴露巢穴位置”这两种风险。本研究证实，除灵长类动物外，其他物种的信号传递决策同样会受到信号接收者易受攻击性的影响。