Meeting Abstract
Owls, Nightbirds (nocturnal members of Caprimulgiformes) and certain hawks have all evolved feather features that reduce the sound they produce in flight. These features are: a leading edge comb (owls) which reduces sound primarily below 10 kHz; vane fringes (owls and nightbirds), which may reduce low or high-frequency sound; and a velvet on the dorsal surface of their wing and tail feathers (all three groups) which ameliorates broadband rubbing sound that includes substantial ultrasound (sound as high as 50 kHz). Two non-mutually exclusive hypotheses make predictions about why silent flight has evolved. The self-masking (“owl ear”) hypothesis states that wing sounds reduce the predator’s ability to hear prey sounds, thus silent flight enables the predator to better locate prey. The stealth (“mouse ear”) hypothesis states that wing sounds reduce the ability of prey to hear the predator approach, limiting the prey’s ability to take evasive action in response to an attack. For owls, which hunt by ear, most available lines of evidence better support the self-masking hypothesis, such as: increases in background sound reduce owl hunting success. For nightbirds, the data better support the stealth hypothesis, as nightbirds do not use sound to hunt and their wing features reduce ultrasound not audible to them but audible to their insect prey. For hawks the answer is unclear; while all hawks use visual cues to hunt, some hawks also use acoustic cues, and not enough is known about either the use of sound nor the distribution of silencing features within Accipitriformes to draw firm conclusions about their function.
