Clark Christopher J, LePiane Krista, Liu Lori
Department of Evolution, Ecology, and Organismal Biology, University of California-Riverside, 900 University Avenue, Riverside, CA 92521, USA.
Integr Org Biol. 2020 Jan 20;2(1):obaa001. doi: 10.1093/iob/obaa001. eCollection 2020.
We raise and explore possible answers to three questions about the evolution and ecology of silent flight of owls: (1) do owls fly silently for stealth, or is it to reduce self-masking? Current evidence slightly favors the self-masking hypothesis, but this question remains unsettled. (2) Two of the derived wing features that apparently evolved to suppress flight sound are the vane fringes and dorsal velvet of owl wing feathers. Do these two features suppress aerodynamic noise (sounds generated by airflow), or do they instead reduce structural noise, such as frictional sounds of feathers rubbing during flight? The aerodynamic noise hypothesis lacks empirical support. Several lines of evidence instead support the hypothesis that the velvet and fringe reduce frictional sound, including: the anatomical location of the fringe and velvet, which is best developed in wing and tail regions prone to rubbing, rather than in areas exposed to airflow; the acoustic signature of rubbing, which is broadband and includes ultrasound, is present in the flight of other birds but not owls; and the apparent relationship between the velvet and friction barbules found on the remiges of other birds. (3) Have other animals also evolved silent flight? Wing features in nightbirds (nocturnal members of Caprimulgiformes) suggest that they may have independently evolved to fly in relative silence, as have more than one diurnal hawk (Accipitriformes). We hypothesize that bird flight is noisy because wing feathers are intrinsically predisposed to rub and make frictional noise. This hypothesis suggests a new perspective: rather than regarding owls as silent, perhaps it is bird flight that is loud. This implies that bats may be an overlooked model for silent flight. Owl flight may not be the best (and certainly, not the only) model for "bio-inspiration" of silent flight.
（1）猫头鹰无声飞行是为了隐身，还是为了减少自我掩蔽？目前的证据略微支持自我掩蔽假说，但这个问题仍未解决。（2）猫头鹰翅膀羽毛的羽片边缘和背部绒羽这两个明显进化来抑制飞行声音的衍生翅膀特征，是抑制空气动力噪声（气流产生的声音），还是相反地减少结构噪声，比如飞行过程中羽毛摩擦产生的摩擦声？空气动力噪声假说缺乏实证支持。相反，有几条证据支持绒羽和羽片边缘减少摩擦声的假说，包括：羽片边缘和绒羽的解剖位置，它们在容易相互摩擦的翅膀和尾部区域最为发达，而非暴露于气流的区域；摩擦的声学特征，其为宽带且包括超声波，在其他鸟类飞行中存在但猫头鹰飞行中不存在；以及其他鸟类飞羽上发现的绒羽与摩擦小羽支之间的明显关系。（3）其他动物是否也进化出了无声飞行？夜鸟（夜鹰目夜行成员）的翅膀特征表明它们可能独立进化出了相对安静地飞行，不止一种昼行鹰（鹰形目）也是如此。我们推测鸟类飞行有噪声是因为翅膀羽毛天生易于相互摩擦并产生摩擦噪声。这个假说提出了一个新观点：与其认为猫头鹰飞行无声，或许鸟类飞行本身就是嘈杂的。这意味着蝙蝠可能是无声飞行一个被忽视的模型。猫头鹰飞行可能不是无声飞行“生物启发”的最佳（当然也不是唯一）模型。
