A study of the silent flight of the owl

Abstract

Many species of owl, including the barn and barred owl, use both visual and bi-aural location to search for prey around dusk and at night. Their bi-aural location system has a maximum sensitivity between 3-6kHz although the hearing of the owl has an upper limit in excess of 20kHz. Its prey, typically voles and mice, squeak and and squeal in the frequency range of 3-6kHz and this range of frequency includes the rustling of leaves made by prey. The hearing of these prey is acute between 2-20kHz. The owl in both gliding and flapping flight generates noise at low frequencies below 2kHz, but is almost totally silent at frequencies above 2kHz. Hence the flight of the owl is almost silent to its prey. When an owl attacks its prey from its perch from a height of 6-10m the prey are unaware of its approach before they are captured by its long talons. The noise suppression devices developed by owls during their long evolution period of millions of years have been identified. They comprise (i) leading edge feathers in the form of a comb, (ii) trailing edge feathers in the form of a fringe, and (iii) fluffy down on the wings and legs. The paper discusses the aerodynamic characteristics of each of these devices and suggests tentative explanations as to how these flow characteristics lead to a large reduction in the noise generated by the owl in flight and especially in its critical range of frequencies above 2kHz. Many species of owl have perfected their use of their habitat and their means of survival through the capture of their prey, in the form of small animals such as voles and mice. The owl as we observe it today has been in existence for over 12 million years. It is suggested it developed its strategy of a silent predator when the small animals were in glut as a result of an abundance of bushes carrying red berries some 10 million years ago. The strategy used by many species of owls involves capturing their prey by day and night and especially at dusk from a perch at a height of 3-6m by gliding steeply towards the prey in daylight or in flapping flight after dark. Some owls today use different techniques for capturing prey. Here we will discuss the 'noiseless' flight of owls such as the barn owl (Tyto alba) and the barred owl (Strix varia), both of which use the strategies discussed above. In the aural response of the prey the owl's flight is completely silent to within 1m of the ground. At this height, even if the approach of the owl is heard, the prey have no time to escape before being struck by the owl's talons. The owl has a highly sensitive bi-aural hearing and location system enabling it to both hear the movements of the prey and its squeaks and squeals together with the rustling of leaves as well as its location. The frequency of these squeaks and squeals and rustling of leaves is in the range 2-10kHz. The maximum hearing response of the bi-aural and location