**2. Eyes**

Arguably the most distinctive feature of owls is their large forward-facing eyes. Instead of the usual 'disc'-shaped eyes normally found in birds, owls have developed large 'tubular'-shaped eyes (**Figure 2**) that are held tight within the orbit and protected by the *scleral ossicles*, or 'sclerotic ring', composed of a series of small interlocking bones that form a bony ring within the sclera. These 'tubular' eyes are so large that in some small species of owl they can take up to 50% of the skull area. Some eagle species have developed 'tubular' eyes, but these are much shorter in length than an owl's, while the majority of diurnal raptors have much smaller 'globose' eyes (**Figure 3**) [2].

The owl's large, forward-facing eyes allow for considerable binocular vision, giving an excellent, but a fairly narrow field of view of 110 degrees, with an overlap of

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

#### **Figure 2.** *Diagram of an owl's eye. Image credit: owlpages.com*

approximately 70 degrees (man, by comparison, has a field of view of 180 degrees with an overlap of 140 degrees) [3]. With such a narrow field of view, many species resort to that very distinctive behaviour of owls, head bobbing, to accurately judge distance and position.

The eyes have extremely large cornea (the transparent outer coating of the eye) and pupil (the opening at the centre of the eye). A larger cornea allows for a larger pupil size, which in turn serves to increase the number of photons that reach the retina (light-sensitive tissue on which the image is formed), thereby improving visual sensitivity [4]. The pupil's size is controlled by the iris (the coloured membrane suspended between the cornea and lens). When the pupil is larger, more light passes through the lens and onto the large retina. Light sensitive retinal cells act as receptors and form images. These receptors are made up of two types of cells, rods and cones; so named for their shapes. Cones distinguish colours, function in bright light, and are needed for sharp resolution, while rods function in low light or at night and

#### **Figure 3.**

*Eye orbit comparison nocturnal-diurnal. L—Little owl (*Athene noctua*). R—Kestrel (Falco tinnuculus). Photo: Alan Sieradzki.*

#### **Figure 4.**

*The nictitating membrane of the Eurasian eagle owl (*Bubo bubo*). Photo: Bruce Marcot.*

are sensitive to movement. Primarily a nocturnal predator, an owl's eyes are packed with rods, giving owls excellent nocturnal vision without the need of the *tapetum*, a reflecting layer at the back of the eye found in most nocturnal animals, including those other nocturnal birds, the Caprimulgiformes. While the ambient light on a cloudy, moonless night rarely drops below an illumination level of 0.004-foot candles, experiments have shown that some species, such as Long-eared Owl (*Asio otus*), Tawny Owl (*Strix aluco*), and Barn Owl (*Tyto alba*), can spot and approach prey from a distance of 6 feet or more under illumination as low as 0.000,00073 foot candles [5]. A recent study's findings indicate that owls may have independently evolved a DNA packaging mechanism in the retina that enhances light channelling in photoreceptors, a feature that has not been observed in any other bird species to date [1].

But an owl's eyes also contain enough cones to enable it to see perfectly in daylight; owls are by no means blind in the daylight. In fact, with its wide range of pupil adjustment, an owl's ability to see sharply is as developed as in any diurnal raptor and has allowed many owl species, such as the Burrowing Owl (*Athene cunicularia*), the Snowy Owl (*Bubo scandiacus*) and the Short-eared Owl (*Asio flammeus*), to become highly successful diurnal hunters. To protect their eyes, owls are equipped with three eyelids. They have a normal upper and lower eyelid, the upper closing when the owl blinks (as in humans—the only bird to do so), and the lower closing up for sleep. The third eyelid is called a *nictitating membrane*, which is a thin layer of tissue that closes diagonally across the eye, from the inside to the outside and cleans and protects the surface of the eye (**Figure 4**).
