Avian eyes and surrounding structures differ from those of mammals in several ways. The sense of sight is extremely well developed in birds; therefore the globe (eyeball) itself is significantly larger than that of equivalent sized mammal. The iris is also much larger, filling the entire area of the open eyelids; the sclera, or white portion of the globe visible outside of the iris, is not normally visible. The iris is highly pigmented. In some species of birds, the iris is pigmented with a red or red-brown color, and in these species, a red eye is normal, as long as it is just the iris that appears red.
Often, a mammal's eye will appear red when blood vessels are engorged and visible in the sclera, what we call "blood-shot" eyes. Since the sclera is not normally visible in birds, this is not a common cause of red eye.
Most birds with red eyes have inflammation of the conjunctiva (mucous membrane surrounding the globe), nictitating membrane (third eyelid) or the eyelids themselves. When the nictitating membrane closes, it moves from the upper half of the medial canthus (area of the eye closest to the beak) toward the lower, outside portion of the eye (lateral canthus). If swelling and redness of the nictitating membrane occurs, the red coloration will appear in the medial canthus.
Redness and swelling of the conjunctiva can appear around the circumference of the eye, although it is usually most noticeable on the lower eyelid margin. In pet birds, the lower eyelid moves up when the eye closes. Swelling and redness of the eyelids are also common. These birds usually appear to be in pain, and may hold the affected eye closed.
The avian skull is full of air pockets or sinuses. These sinuses reduce the overall weight of the skull – a necessary adaptation for flight. The sinuses connect with the nasal passages and extend completely around the eyes. Because of this connection, respiratory infections often extend into the sinusitis and periocular tissues. Birds with sinusitis often have swelling surrounding the eyes, due to inflammation of the sinus wall or because the sinus itself is full of exudate (pus). If large amounts of exudate accumulate, the eye may actually be pushed forward, bulging out from the skull.
Inside of the globe itself is a vascular structure, called the pecten, which supplies nutrients to the retina. The pectin is not visible without specialized magnification, because it is located behind the iris. Severe trauma to the globe or infection within the globe itself (inside the eyeball) may cause bleeding within the globe (intraocular hemorrhage). Intraocular hemorrhage is termed hyphema and appears as a solid red eyeball, with no visible pupil.
Inflammation of the conjunctiva may occur due to environmental irritants, such as dust, aerosol sprays or smoke. Birds are extremely sensitive to cigarette smoke, and smoking around birds should be avoided at all times. If possible, eliminate the amount of dust the bird is exposed to. If ocular redness persists for over 24 hours, or if other symptoms are present, veterinary attention is necessary.