- The avian eye is both relatively and absolutely large relative to its body size.
- The sclera is invested with a ring of small bones anteriorly and cartilage posteriorly.
- All birds have some degree of striated muscle in their iris making the use of parasympatholytic agents (eg atropine) ineffective in bringing about dilation.
- Birds lack a choroidal tapetum.
- The avascular retina is nourished by the externally situated choroid and the internally projecting pecten. The pecten is a heavily pigmented, highly vascularized, pleated structure that projects into the vitreous from a base situated upon the optic nerve. The pecten limits visualization of the optic nerve to a thin rim a the base of the pecten.
A bird is a wing guided by an eye… Rochon-Duvigneaud: Lex Yeux et La Vision Des Vertebres
The avian eye is a large structure that takes up a significant portion of cranial mass. Raptors depend heavily on vision in order to compete successfully for survival.
The posterior aspect of the eye fits snugly within the large bony orbit. The globes are separated by a thin interorbital septum, which measures significantly less than 1 mm in some areas. The interorbital septum is completely bony in some species and partially fibrous in others. Much of the lateral aspect of the globe lies outside of the protection provided by the bony orbit.
The shape of the raptor eye varies with the species and is determined by the shape and extent of the scleral ossicles. The eye is relatively flat in some small diurnal raptors, such as the kestrel (Falco tinnunculus), globoid in large diurnal raptors (e.g. buteos), and tubular in owls.
Ocular mobility is limited in birds. There is no retractor oculi muscle, and the two oblique and four rectus extraocular muscles are relatively thin. Pyramidalis and quadratus muscles, that take origin from the posterior surface of the globe, act in coordination with one another to move the nictitans.
The sclera is not readily observed in most birds because of the tight fit of the eyelids at the limbus. The scleral ring is composed of 10-18 (15 in most raptors) small interdigitating bones and invests the anterior portion of the sclera. Ossicles determine eye shape (Fig 1), provide protection for internal ocular structures, and serve as the site of origin for striated ciliary muscles.
Avian eyelids are thin and sometimes translucent. The lids are sparsely feathered, although the margin has delicate filoplumes in most species. The palpebral margin is usually pigmented and lacks Meibomian glands. The lower lid covers the majority of the cornea and it contains a fibroelastic plate (tarsus), which is roughly semicircular in most birds of prey.
The nictitating membrane is well developed in all birds. The nictitans is translucent to opaque and generally possesses a pigmented leading edge. The nictitans spreads the tear film across the cornea, moving obliquely from dorsonasal to ventrotemporal in most raptors. Contracture of the pyramidalis and quadratus muscles controls nictitans movement.
The layers of the raptor cornea are the:
- Anterior corneal epithelium
- Anterior limiting lamina (Bowman’s layer)
- Substantia propria (stroma comprising the majority of corneal thickness)
- Posterior limiting lamina (Descemet’s membrane)
- Posterior epithelium (endothelium)
The avian iris contains variable amounts of striated muscle, smooth muscle, and a myopepithelial dilator muscle. Circumferential striated muscle serves as the primary pupillary sphincter in all species examined rendering the use of parasympatholytics (e.g. atropine, tropicamide) ineffective in bringing about dilation.
Iris vasculature is extensive, but may be obscured by stromal elements, lipochrome or pteridine pigments. Iris color may suggest the relative age of an individual in certain species. The juvenile red-tailed hawk has a yellow-gray iris, which becomes darker with age until it achieves a deep chocolate-brown by four years. The accipiter iris is gray in nestlings, yellow in juveniles and young adults, orange in middle-aged adults, and ruby red in older adults (5 years).
The iridocorneal angle is a well-developed region in raptors, most extensive in owls. This region includes the most posterior internal aspect of the cornea, the most anterior external aspect of the ciliary body, and the root of the iris. Trabeculae that constitute the pectinate ligament may be seen crossing the iridocorneal angle as they pass from the cornea to the iris (Fig 2).
Ciliary muscles are striated in birds, allowing a swift mechanism for visual accommodation. The ciliary processes can be visualized as numerous club-shaped processes that attach directly to the lens capsule near the lens equator.
The avian lens is relatively large and generally softer in birds than in mammals. This soft lens allows extremely fast visual accommodation, and makes phacoemulsification an ideal method for lens removal. An annular pad lies directly under the lens capsule in the region of ciliary process attachment. The annular pad is much more extensive in diurnal raptors than in owls.
The choroid is well developed in birds. Choroidal vasculature is clearly visible in most young owls, which have a paucity of pigment in the retinal epithelium (Fig 3). Choroidal vessels are harder to appreciate in most diurnal raptors and in many cases are totally obscured by the overlying pigmented retinal epithelium.
Birds lack a choroidal tapetum, although a retinal pigmented epithelial tapetum has been described in caprimulgiforms such as the whip-poor-will (Caprimulgus vociferus).
The pecten is a heavily pigmented, highly vascularized, pleated structure that projects into the vitreous from a base situated on the optic disk-which it largely obscures (Fig 4). The pecten is typically located ventrotemporal within the eye, and is more developed in diurnal raptors than owls. Numerous functions have been ascribed to this structure with most current investigators agreeing on the provision of nutritional support to the inner aspect of the avascular retina, assistance in maintaining intraocular acid-base balance and participating with fluid mixing in the posterior segment of the eye. Fluorescein angiography in owls has revealed frequent, forceful ejections of blood from the pecten into the vitreous, which seems to support the theory of retinal nutritional support.
The avascular retina is nourished primarily by the underlying choroid. The presence and number of fovea are species-dependent. Diurnal raptors have 2 foveae in each eye, one located centrally and the other temporally. Owls possess a single temporal fovea that appears as a depression dorsotemporal to the pecten. The foveae play an important role in visual acuity. Recent studies have shown the two fovea of diurnal raptors coordinate with each other, creating an extensive area of high visual acuity (approximately 50 degrees in the red-tailed hawk).