Two hypotheses have been proposed for the evolution of structures that reduce flight sounds in birds. According to the stealth hypothesis, flying quietly reduces the ability of other animals (e.g. prey) to detect the animal's approach from its flight sounds. This hypothesis predicts that animals hunting prey with acute hearing evolve silencing features. The self-masking hypothesis posits that reduced flight sounds permit the animal itself to hear better (such as the sounds of its prey, or its own echolocation calls) during flight. This hypothesis predicts that quieting features evolve in predators that hunt by ear, or in species that echolocate. Owls, certain hawks and nightbirds (nocturnal Caprimulgiformes) have convergently evolved a sound-reducing feature: a velvety coating on the dorsal surface of wing and tail feathers. Here we document a fourth independent origin of the velvet, in the American Kestrel (Falco sparverius). Among these four clades (hawks, falcons, nightbirds, owls), the velvet is longer and better developed in wing and tail regions prone to rubbing with neighboring feathers, apparently to reduce broadband frictional noise produced by rubbing of adjacent feathers. We tested whether stealth or self-masking better predicted which species evolved the velvet. There was no support of echolocation as a driver of the velvet: Oilbird (Steatornis caripensis) and Glossy Swiftlet (Collocalia esculenta) each evolved echolocation but neither had any velvet. A phylogenetic least squares fit of stealth and self-masking (to better hear prey sounds) provided support for both hypotheses. Some nightbirds (nightjars, potoos and owlet-nightjars) eat flying insects that do not make much sound, implying the velvet permits stealthy approach of flying insects. One nightbird clade, frogmouths (Podargus) have more extensive velvet than other nightbirds and may hunt terrestrial prey by ear, in support of self-masking. In hawks, the velvet is also best developed in species known or suspected to hunt by ear (harriers and kites), supporting the self-masking hypothesis, but velvet is also present in reduced form in hawk species not known to hunt by ear, in support of the stealth hypothesis. American Kestrel is not known to hunt by ear, and unlike the other falcons sampled, flies slowly (kite-like) when hunting. Thus the presence of velvet in it supports the stealth hypothesis. All owls sampled (n = 13 species) had extensive velvet, including the Buffy Fish-owl (Ketupa ketupu), contrary to literature claims that fish-owls had lost the velvet. Collectively there is support for both the self-masking and stealth hypotheses for the evolution of dorsal velvet in birds.

中文翻译：

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夜鸟、老鹰、猎鹰和猫头鹰安静飞行的进化和生态相关性

对于减少鸟类飞行声音的结构的进化，已经提出了两种假设。根据隐身假说，安静地飞行会降低其他动物（例如猎物）从其飞行声音中发现动物接近的能力。该假设预测，具有敏锐听力的猎物动物会进化出沉默特征。自我掩蔽假说认为，减少的飞行声音可以让动物在飞行过程中更好地听到（例如猎物的声音，或它自己的回声定位呼叫）。这个假设预测安静的特征在靠耳朵捕食的捕食者或回声定位的物种中进化。猫头鹰、某些鹰类和夜鸟（夜间活动的猫头鹰目）已经逐渐进化出一种降低声音的特征：翅膀和尾羽的背面有一层天鹅绒般的涂层。在这里，我们记录了天鹅绒的第四个独立起源，即美洲红隼 (Falco sparverius)。在这四个进化枝（鹰、猎鹰、夜鸟、猫头鹰）中，天鹅绒更长，在易与相邻羽毛摩擦的机翼和尾部区域发育得更好，显然是为了减少相邻羽毛摩擦产生的宽带摩擦噪音。我们测试了隐身或自我掩饰是否能更好地预测哪个物种进化出了天鹅绒。没有支持回声定位作为天鹅绒的驱动因素：油鸟（Steatornis caripensis）和光泽金丝燕（Collocalia esculenta）都进化出回声定位，但都没有任何天鹅绒。隐身和自我掩蔽（以更好地听到猎物的声音）的系统发育最小二乘拟合为这两种假设提供了支持。一些夜鸟（夜鹰，potoos 和 owl-nightjars）吃不会发出太多声音的飞虫，这意味着天鹅绒允许飞虫偷偷接近。夜鸟的一个分支，蛙嘴鸟（Podargus）比其他夜鸟有更广泛的天鹅绒，可能会用耳朵捕食陆地猎物，以支持自我掩饰。在鹰类中，天鹅绒在已知或怀疑用耳朵捕食的物种（鹞和风筝）中发育得最好，支持自我掩蔽假说，但天鹅绒也以减少的形式存在于不知道用耳朵捕食的鹰类中，在支持隐身假说。众所周知，美洲红隼不会用耳朵捕猎，而且与其他采样的猎鹰不同，它们在捕猎时飞得很慢（像风筝一样）。因此，其中天鹅绒的存在支持了隐形假设。采样的所有猫头鹰（n = 13 种）都有大量的天鹅绒，包括巴菲鱼鸮 (Ketupa ketupu)，这与文献中声称鱼鸮失去了天鹅绒的说法相反。总的来说，鸟类背部天鹅绒进化的自我掩蔽和隐身假说都得到了支持。