Female-only colour polymorphism is rare in birds, but occurs in brood parasitic cuckoos (Cuculidae). Obligate brood parasites leave incubation and parental care to other species (hosts), so female-female interactions can play a role in how parasites guard critical resources (host nests) within their laying areas. The plumage of adult female common cuckoos (Cuculus canorus) is either rufous (typically rare) or grey (common), whereas adult male conspecifics are monochromatic (grey). In previous studies, hosts and conspecific males responded with less intensity toward the rare female morph in support of a negative frequency-dependent benefit of female plumage polychromatism. Here, we assessed responses of both conspecific females and males to vocal playbacks of female calls, coupled with one of two 3D models of the different morphs of female cuckoos. At our study population, the rufous female morph was as common as the grey morph; therefore, we predicted similarly high rates of conspecific responses in both treatments. Both female and male cuckoos responded to playbacks acoustically, which demonstrated the primary role of acoustic communication in social interactions amongst cuckoos. Following this, some cuckoos flew closer to the models to inspect them visually. As predicted, no significant differences were detected between the live cuckoos’ responses toward the two colour morphs in this population. We conclude that dichromatism in female cuckoos evolved to serve one or more functions other than conspecific signalling.

中文翻译：

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女性对女性的攻击和男性对女性常见杜鹃的两种颜色形态的反应。

仅雌性的颜色多态性在鸟类中很少见，但在雄性寄生杜鹃（Cuculidae）中发生。专性的母体寄生虫将孵化和父母照料留给其他物种（寄主），因此雌/母之间的相互作用可以在寄生虫如何保护产卵区的关键资源（寄主巢）中发挥作用。成年雌性杜鹃（Cuculus canorus）的羽毛）是红褐色的（通常是罕见的）或灰色的（常见的），而成年雄性的同种是单色的（灰色的）。在以前的研究中，寄主和同种雄性对稀有雌性变体的反应强度较小，以支持雌性羽毛多色性的负频率依赖性。在这里，我们评估了同种雌性和雄性对雌性声音的声音回放的响应，以及雌性杜鹃形态不同的两个3D模型之一。在我们的研究人群中，红褐色的雌性变体和灰色的变体一样普遍。因此，我们预测两种治疗方法中的同种异体反应的发生率相似。雌性杜鹃和雄性杜鹃都对回声做出了反应，这表明了杜鹃之间社交互动中声音交流的主要作用。按照此，一些杜鹃飞到模型附近，以目视检查它们。如所预测的，在该种群中，杜鹃对两种颜色形态的反应之间没有发现显着差异。我们得出的结论是，母杜鹃中的重色性进化为具有一种或多种功能，而不是特定的信号传导。