Topics in medicine and surgery
Avian Vision: A Review of Form and Function with Special Consideration to Birds of Prey

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Abstract

More so than any other terrestrial vertebrate, birds rely most heavily on their ability to assess their visual environment. High visual acuity is not only necessary to find and acquire food, but also to navigate surroundings, to identify conspecifics and potential mates, and to quickly identify and escape from predators. This article aims to help the reader understand how birds, particularly birds of prey, see by reviewing pertinent anatomy and physiology of the eye, color vision, visual fields, visual acuity, accommodation, and flicker-fusion frequency. For more information on specific disease conditions of the avian eye, the reader is encouraged to review any number of avian medical and surgical texts.

Section snippets

The Orbit

The orbit is formed by the frontal, prefrontal, sphenoid, ethmoid (mesethmoid), palatine, quadrate bones, and the jugal arch.3, 4, 5, 6, 7 The dorsal and temporal aspects of the globe are unprotected by the bony orbit, but the rest of the globe fits very snugly within the orbit. In woodpeckers, the tight fit of the globe within the orbit, in addition to extensive fascial connections between the orbital rim of the globe and the narrowed orbital entrance compared with the size of the posterior

Extraocular Muscles

Extraocular muscles in avian species are reduced to thin bands, and torsional movement of the globe in many species is limited to between 2° and 5°.9 These limitations are due to the largeness of the globe and its fit within the orbit. The extraocular muscles consist of the medial, lateral, dorsal, and ventral rectus muscles, and the dorsal and ventral oblique muscles. Eye movements, which are of small amplitude, have been described as (1) impulses and oscillations, (2) tremors, (3) flicks, and

Nictitating Membrane

The nictitating membrane lies in the dorsonasal quadrant of the conjunctival sac. Movement of this elastic, thin membrane over the surface of the cornea is swift, except in owls (Strigiforms).7 In domestic fowl, the nictitating membrane moves across the cornea 30 to 35 times a minute.3 It is covered by a papillary layer of epithelium and serves to protect, moisten, and clean the cornea.11, 12 The free margin of the membrane, called the Plica marginalis, marginal plait/flange, or marginal fold,

Fibrous Tunic (Cornea and Sclera)

Similar to that of other vertebrates, the avian cornea is transparent and avascular, and makes up the anterior portion of the fibrous tunic of the globe. Globular or tubular eyes, like those of the eagle or owl, respectively, have a strongly curved cornea with a small relative area compared with the rest of the globe. The cornea functions to support the intraocular contents, refract light by its curvature, and transmit light by its transparency. The avian cornea consists of 5 layers but is

Vascular Tunic (Uvea)

The iris, ciliary body, and choroid make up the vascular tunic or uvea of the globe. Originating from the anterior portion of the ciliary body, the iris extends centrally and makes a diaphragm in front of the lens, thus creating an anterior and posterior chamber with communication via the pupil. The iris, which consists of numerous blood vessels, fibroblasts, nerves, collagen, epithelial cells, and an extensive musculature component, regulates the amount of light that enters the posterior

The Lens

The anatomy of the avian lens differs greatly from that of mammals in that it has an annular pad (pulvinus annularis, ringwulst, or borrelet) around its central core.7 The annular pad is separated from the central core by a fluid-filled chamber, vesicula lentis.7 This anatomical arrangement may be a hydrostatic mechanism for transmitting pressure from the ciliary muscle to the central core to facilitate accommodation (see “Accommodation”).7 The avian lens, along with the cornea, serve to

Nervous Tunic (Retina)

The retinal layers of birds are the same as those of other vertebrates; however, some variations exist in morphology, areas of visual acuity, and retinal vascularization. Cones function in sharp visual acuity, color, and daylight vision. They are present within the retina as a single cell or in a pair (e.g., double cone). The double cone consists of a longer, principal cone that contains an oil droplet and an adjacent, smaller accessory cone with a paraboloid (e.g., barrel-shaped body of

The Eyelids

The upper eyelid of birds is short and thick, whereas the lower eyelid is thin, longer, and more mobile because it contains a fibroelastic tarsal plate and is responsible for closing the eye.3 The palpebrae are devoid of meibomian glands and may be feathered or unfeathered. Eyelid movement is controlled by 4 striated muscles. The levator palpebrae superioris muscle, which has both skeletal and smooth muscle fibers, elevates the upper eyelid, whereas the depressor palpebrae ventralis muscle, as

Nervous Innervation of the Eye and the Periorbita

Because birds are exquisitely visual animals, it is important to understand nervous innervation of the eye and its associated structure. The optic nerve (cranial nerve [CN] II) is most highly developed in falconiformes and is less developed in nocturnal avian species.44 Afferent fibers from the ganglion cells of the retina become myelinated as they enter the sclera and course caudally through the optic foramen.44 At this point, nearly all of the fibers decussate at the top of the optic chiasm,

Color Vision

The addition of color perception to an animal’s visual world enhances visual activity by adding an extra element of contrast, which helps the animal distinguish among different objects in the environment. The affective aspects of color perception and the heightened sense of awareness and attention that can improve an animal’s ability to attain food, become attracted to potential mates, and assess threats are just as important. Research in avian ocular physiology and behavior over the past

The Fovea

Some animals possess specialized regions within the retina capable of producing visual acuity far greater than in the retina outside those regions. Such regions (termed visual streaks, areae centralis, or simply areae) owe their elevated acuity mainly to higher photoreceptor and ganglion cell densities.64 A fovea (Latin for “small pit”) is a small, subspecialized portion in the center some of these areas that is designed to subserve the highest possible visual acuity. Design features that

Visual Fields and Visual Acuity

The position of the eye is one of the most important determinants of the field of vision in avian species. Because most birds have very limited movement of the eye within the socket, they must rely on movement of the head and neck to examine their surrounding visual environment. It is also important to recall that most birds are bifoveate, have 2 functionally separate visual fields, and have high visual acuity both in front and to their sides without the necessity of moving their heads to any

Image Stabilization and Visual Acuity

To maintain visual acuity, the eyes must be stabilized relative to the object of interest.49 The ability to maintain a stable visual image is of extreme importance while birds, such as raptors, are attempting to capture prey in flight and vice versa when prey species are attempting to avoid capture. The principal threat to ocular stability and visual acuity is motion of the head and neck while trying to maintain fixation on an object.49 Birds attempt to stabilize their gaze and avoid “retinal

Accommodation

Accommodation is the ability to focus the eye and increase refractive power to see objects sharply at varying distances, and is one of the most important mechanisms of achieving high visual resolution in animal species.1 Accommodation can be an active (dynamic) or passive (static) process and is largely determined by optical parameters of the eye such as retinal image size and image resolution, as well as by the environment and lifestyle of the animal.19 Birds have accommodative mechanisms that

Corneal Accommodation

The image formed on a bird’s retina is a function of both corneal and lenticular accommodative mechanisms. Bird eyes are considered to be emmetropic (condition of the normal eye when parallel rays are focused exactly on the retina and images are in clear focus) when on land, and, as a result, the cornea plays an important role in accommodation.19, 107, 108 In fact, the cornea is responsible for approximately two thirds of the refractive power and range of accommodation of the avian eye because

Lenticular Accommodation

Lenticular accommodation occurs primarily through 2 mechanisms. First, the posterior sclerocorneal muscle brings about accommodation by forcing the ciliary body directly against the lens rather than indirectly through the zonular (or suspensory) ligaments, resulting in increased curvature of the lens (primarily in diurnal birds).3, 19 The direct articulation between the ciliary body and the lens occurs through extension of the equatorial diameter of the lens by elongation of the equatorial

Accommodation and Age

As in humans, the ability to accommodate is expected to decrease with age (presbyopia). These changes as a function of age have been well studied in chickens.20, 106, 121, 122, 123 Electrical stimulation of the eye and the Edinger-Wesphal nucleus, use of a pharmacologic agent to stimulate accommodation, and use of a suction electrode to stimulate the ciliary nerve and ganglion in excised chicken eyes of various ages are techniques that have been used to evaluate presbyopia in the avian eye.106,

Flicker-Fusion Frequency

In avian species, the ability to detect and discern or resolve rapid movements may be the difference between life and death in regards to rapid flight, the capture of prey, or the act of becoming prey. The ability to resolve rapid movements, or the frequency at which motion can no longer be resolved, is known as the flicker-fusion frequency (FFF).2, 125, 126 Because temporal resolution is determined primarily by photoreceptor density and neural complexity, it is understandable that the ability

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