**4. Hearing**

Owls have a unique, complex and highly developed, and specialised auditory system designed to aid in the location and capture of prey. Most owls use a combination of their remarkable hearing and eyesight to locate and capture their prey. However, some species, such as the Barn Owl (*Tyto alba*), the Great Grey Owl (*Strix nebulosa*), the Long-eared Owl (*Asio otus*) and the Short-eared Owl, can use their unique auditory powers to locate and seize prey invisible to the eye and hidden in thick vegetation or even under a deep covering of snow [12]. The facial plumage of the owl forms a parabolic dish, or facial disc, edged by a ruff, that focuses and enhances sounds received (**Figure 5**). The ears are located at the sides of the head, behind the eyes, and are covered by the densely packed auricular feathers of the facial disc and ruff (**Figure 6**). The size and shape of the ear opening vary from species to species, with some species also having either a pre-aural or postaural operculum or flap (**Figure 7**).

An owl's range of audible sounds is not unlike that of humans, but its hearing is exceptionally more acute within certain frequencies; particularly at frequencies of 5 kHz and above [13], maximising hunting accuracy with frequencies between 4 and 8 kHz [14]. Some owl species have asymmetrically set ear openings (i.e. one ear is higher than the other). This asymmetry is found in five phyletic lines, represented in the Genera *Tyto, Phodilus, Strix, Rhinoptynx, Asio, Pseudoscops*, and *Aegolius* [15]. Ear asymmetry makes the auditory directional sensitivity pattern for high frequencies different in elevation between the two ears. This allows the owl to localise sound in the vertical plane, by comparing the intensity and spectral composition of sound between the two ears. In simple terms, when a noise is heard, the owl can locate its source because of the minute time difference in which the sound is perceived in the left and right ears. This interaural time difference can be as short as 10 millionths of

#### **Figure 5.**

*Perfect facial disc of the great Grey. Owl (*Strix nebulosa*). Photo: Tony Hisgett. Source: Wiki commons: https:// creativecommons.org/licenses/by/2.0/deed.en*

*Designed for Darkness: The Unique Physiology and Anatomy of Owls DOI: http://dx.doi.org/10.5772/intechopen.102397*

#### **Figure 7.**

*Operculum of a long-eared owl (*Asio otus*). Photo credit: Creative commons'—https://creativecommons.org/ licenses/by-sa/2.5/deed.en*

a second [16]. For example, if the sound was to the left of the owl, the left ear would hear it before the right ear. The owl then turns its head until the sound arrives at both ears simultaneously—the prey, even when not visible due to darkness or cover, is now directly in the owl's line of sight. Even once the prey has been located and the owl has launched an attack, the owl will continue to make minute adjustments of the moveable ruff and flaps until the moment of strike. To understand how this works, research has turned to the area of neurology.

Recent research has discovered that interaural time differences (ITD) are used to localise sounds in azimuth, whereas interaural level differences (ILD) are used to localise sounds in elevation. These two features are processed independently in two separate neural pathways that converge in the external nucleus of the inferior colliculus to form an *auditory map of space* [17]. The brain constructs the auditory space map by comparing the responses of neurons in the two ears to a sound that stimulates both. The left-right positioning of the sound source is computed from the different arrival times of the sound at each ear [18]. Owls with symmetrical ears must determine the horizontal and the vertical directions of a sound separately, one after the other, by tilting head movements [14], thus making it that little bit more difficult a process to lock on to moving prey. Once the prey has been located and locked on to and the owl has launched itself into the attack, movements of the facial ruff and flaps continue to make minute adjustments throughout the flight path until the moment of impact.
