Abstract

Agonistic interactions between obligate avian brood parasites and their hosts can lead to the coevolution at any stage of the nesting cycle, yet adaptations and counter-adaptations at the fledgling stage are poorly known. Young of the host-specialist screaming cowbird (Molothrus rufoaxillaris) closely resemble those of its greyish baywing (Agelaioides badius) host in appearance and begging calls. This overall similarity has shown to be adaptive to escape host discrimination after leaving the nest, but the role of acoustic signals in host deception remained unclear. We examined whether baywing parents are able to distinguish between begging calls of mimetic and non-mimetic fledglings and whether screaming cowbirds can trick host parents by vocally resembling host young. We conducted a field playback experiment using fledgling calls of screaming cowbird (mimetic), shiny cowbird (M. bonariensis; non-mimetic) and baywing (conspecific control) in the absence of any visual stimuli. Baywings were significantly less responsive to non-mimetic shiny cowbird calls than to the other call types and more responsive to screaming cowbird calls than to conspecific calls. The results support the idea that baywings cue in on species-specific acoustic signals for fledgling recognition and that vocal similarity to host young in screaming cowbirds plays a role in host deception. The observed host preference for screaming cowbird over conspecific calls further suggests that vocal mimicry in brood parasites could be reinforced by the expression of acoustic features that either act as a supernormal stimulus or exploit host’s pre-existing sensory biases.

Significance statement

Brood-parasitic screaming cowbird juveniles closely resemble those of its primary host, the greyish baywing, in plumage appearance and vocalizations. Using a field playback experiment, we studied the ability of adult baywings to discriminate against foreign juveniles based on vocal cues, in the absence of any visual stimuli. Baywings responded differentially to own-species and non-mimetic calls, and even more, they were more attracted to screaming cowbird calls than to those of conspecific juveniles. The study supports the idea that baywings use acoustic signals to distinguish between their own and foreign fledglings, thus favouring the evolution of vocal mimicry in its specialist brood parasite. Moreover, the results suggest that screaming cowbird fledglings would be even more effective than host’s own young in attracting parental attention through their begging calls.

中文翻译：

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宿主的声学歧视有利于幼鸟的声音技巧

摘要

专性鸟类孵化寄生虫与其宿主之间的竞争性相互作用可导致筑巢周期的任何阶段的共同进化，但在羽翼未丰的阶段的适应和反适应却鲜为人知。寄主专家尖叫牛鹂 ( Molothrus rufoaxillaris ) 的幼鸟与灰色海湾翅 ( Agelaioides badius ) 的幼鸟非常相似) 主持出现和乞讨电话。这种整体相似性已被证明可以在离开巢穴后适应逃避宿主的歧视，但声学信号在宿主欺骗中的作用仍不清楚。我们检查了baywing父母是否能够区分模仿和非模仿雏鸟的乞求呼叫，以及尖叫的牛鸟是否可以通过与寄主年轻的声音相似来欺骗寄主父母。我们使用尖叫的牛鸟（模仿物）、闪亮的牛鸟（M. bonariensis; 非模拟）和baywing（同种控制）在没有任何视觉刺激的情况下。与其他呼叫类型相比，Baywings 对非模仿闪亮牛鸟呼叫的响应明显较低，并且对尖叫的牛鸟呼叫的响应比对特定呼叫的响应更高。结果支持了这样的观点，即baywings提示物种特异性声学信号以进行初出茅庐的识别，并且在尖叫的牛鸟中与宿主年轻的声音相似性在宿主欺骗中起作用。观察到的宿主更喜欢尖叫的牛鸟而不是同种叫声，这进一步表明，通过声学特征的表达，可以加强孵化寄生虫中的声音模仿，这些声学特征要么充当超常刺激，要么利用宿主预先存在的感觉偏差。

意义陈述

幼体寄生的尖叫牛鹂幼鸟在羽毛外观和发声方面与它的主要宿主灰色的海湾翼非常相似。使用现场回放实验，我们研究了成年湾翅在没有任何视觉刺激的情况下根据声音线索区分外国幼体的能力。Baywings 对自己物种和非模仿叫声的反应不同，更重要的是，它们更喜欢尖叫的牛鸟叫声，而不是同种幼鸟的叫声。该研究支持这样一种观点，即湾翅使用声学信号来区分自己的和外国的雏鸟，从而有利于其专业育雏寄生虫中声音模仿的进化。而且，