Peer into the world of birds, and eyes of many different colors peer back.
There are shades of light and dark - yellows, whites, greens, and the red eyes of certain hawks, ducks, loons, herons, and songbirds. These colors really pop because birds have no white around the iris like we do. So the yellow eyes of a Great Horned Owl or a Herring Gull seem to give the birds a fierce, penetrating glare. If you picture them with soft brown eyes, though, suddenly they seem much less intimidating.
While eye color isn’t tied to one group of birds or another, a pattern common to many birds is a change in eye color as immature birds grow to adulthood. Bald Eagles, Ring-billed Gulls, and ducks such as goldeneyes and scaup have brown eyes as youngsters, and yellow as adults. Red-tailed Hawks reverse this pattern, with their eyes changing from yellow to brown, while the yellow eyes of a young Cooper's Hawk turn deep red as it reaches maturity.
Not all birds’ eyes change color as the birds age. But for those whose eye color appears to signal adulthood, this is likely an adaptation that helps them gauge the maturity – and suitability – of potential mates.
Eye color is even more varied in birds than it is in humans. For example, you don’t hear of people with red eyes. Unless they have conjunctivitis or have been smoking too much grass. Eye color in birds can vary from black to brown to red to orange to yellow to blue to green to white and many colors in between. The color of a bird’s eye, as in the color of a bird’s feather, can be caused by both pigments and refraction of light. In fact, many birds exhibit more pigment coloration in their eyes than humans.
Adult Ospreys have brilliant yellow eyes, but they are not born with this eye color. The Osprey’s eye color changes from blood red in nestlings to orange-yellow in juveniles to yellow in adults. Ospreys, when they hatch, have dark blue eyes.
A bright yellow eye on this Common Grackle is in stark comparison with the muted earth tones of her body.
Eyeshine
The eerie "eyeshine" exhibited by owls and many other animals is a result of the animal's tapetum lucidum. A tapetum lucidum is a layer of tissue behind an animal’s retina that reflects the visible light spectrum. This reflection dramatically increases the light available to the animal’s photoreceptors, and gives it superior night vision. Tapetum can reflect different colors—blue, green, yellow, or, in the case of this owl, yellow.
An eagle’s eyeball is almost the same size as a human eye. Given that the eyeball is so large relative to the size of the head, an eagle’s eyes fill most of the skull. Each eyeball is “fixed” in the skull, held in place by a sclerotic ring. Eagles are unable to move their eyeballs within the socket. Adult Bald Eagles have yellow eyes. Juvenile Bald Eagle eyes are brown and gradually lighten as they mature.
Among ducks, red, yellow and dark eyes dominate. Here’s a Northern Shoveler showing often his many colors and his bright yellow eye. Other common ducks with yellow eyes include the Ring-necked Duck, the Redhead males, the Tufted Duck, the Greater and Least Scaups, the Common and Barrows Goldeneyes, and the Hooded Merganser.
Grebes have a variety of eye colors depending on the species. The Western Grebe and its close relative, the Clark’s Grebe, have bright red eyes, as do the Horned Grebe and Eared Grebe. One of the most common grebes I see regularly is the Pied-billed Grebe which has a black eye with a thin white ring around it. The Red-necked Grebe has a black eye without an eye-ring. Finally, the Least Grebe has a bright yellow eye with a pinpoint black pupil.
First-year Red-tailed Hawks (and most buteos) have pale yellowish eyes and adults have dark brown eyes, but this color change from yellow to brown happens over several years, and transforms more quickly in some birds than in others, and varies by species as well.
Both male and female mature Cooper’s hawks look similar, though the females are generally larger. They share the same proportions and field marks, however, and careful study of these birds will yield easy clues to their identity. Mature Cooper’s hawks have distinct red eyes with dark pupils. Juvenile Cooper’s hawks can be puzzling to identify because they lack the distinct coloration of adult birds, but savvy birders can note a range of clues that will positively identify these birds. The best clue to identify a juvenile bird is the eye color. Young Cooper’s hawks have a yellow eye that will gradually darken to orange and then red as they mature
Moving along to the birds with red eyes, here we have a gorgeous male Wood Duck. I don’t know about you, but I think Wood Ducks are among the most beautiful ducks we have in North America. It has blue, black, white, green, purple, orange, yellow, chocolate brown, light brown and, thanks to that eye, red. An astonishing mix of colors.
Other species with red eyes include the Canvasback, the Cinnamon Teal male, and the Red-breasted Merganser, the Phainopepla male, the White-tailed Kite, and the Black Oystercatcher.
The majority of birds have dark brown or black eyes (too many to mention) but I love this photo I took a couple of years ago and offer it up as an example of a gorgeous dark-eyed raptor.
This male Phainopepla (one of the fun names to pronounce in birding) sports a spiky crest and a bright red eye. Phainopeplas are resident in the West/Southwest, so those of you in the Midwest or Atlantic seaboard may not have seen this bird. The female also carries a red crest and red eye, but she has a brown body.
This Double-crested Cormorant has a bright blue eye that looks like a jewel. When I Googled blue-eyed birds, I came up with the Satin Bowerbird (not a North America native) and a musical rock band with this name based in Connecticut. Since the cormorant looks like a dinosaur-bird, somehow it seems like the name fits a rock band.
The White-tailed Kite is another raptor with a stunning red eye that is sunk in a deep dark socket and you often don’t see it. Most photos of these kites only capture the black sockets and only when the bird is turned at the correct angle in the sun can you see those red eyes. A lovely raptor that hovers in the sky over the patch of ground where it seeks prey, the kite plummets in a dive to grab its meal. It’s an amazing sight to encounter.
Another yellow-eyed stunner, this male Hooded Merganser shows off his greatest feature, his abundant black-and-white crest, which must turn the head of every female who is impressed by his ability to strut while he swims. Nicely done, young fella!
The Cinnamon Teal is one of my favorite ducks. Its deep, rich, cinnamon feathers and striking long feathers on its back provide the perfect landscape for appreciating its fantastic red eye. What a beauty!
Another yellow-eyed everyday bird with its repertoire of joyous calls, the Northern Mockingbird stands alone. I’ve been asked so many times, what bird it is that sings at night with many calls and songs as I’m trying to sleep? I’m sure you’ve heard that question as well. The answer? Most commonly the Northern Mockingbird, although American Robins and Hermit Thrushes also sing at night. Some other birds do as well but they’re less likely to be heard around one’s home.
Here’s another red-eyed beauty, the Spotted Towhee. This bird was formerly known as the Rufous-sided Towhee and I still prefer that name.
Now, let’s look closer at what we know about birds’ eyes and vision. Here’s a scientific paper that helps us understand more about avain eyesight than eye color. So many interesting details in this article!
Why Birds’ Eyes Are So Different From Ours
Birds are highly visual animals with unique features and adaptations that allow them to fly. As they begin to migrate, they use visual cues to help guide them. Their eyes can change focus rapidly using an active process called accommodation. Birds also see ultraviolet light, and they have enhanced visual acuity because of different mechanisms, including a one-to-one projection of receptor cells to ganglion cells in the retina.
Avian Ocular Anatomy & Physiology
Birds are the most visually dependent class of vertebrates. Even though humans are highly visual, the information transmitted to our brains is only 40 percent of that transmitted by pigeons and chickens. Birds of prey have even greater visual acuity. Pigeons can discern subtle color differences, and other avian species are able to record and remember over 6,000 images of caches where food is stored.
Eye position in birds can be lateral in the skull or directed frontally, particularly in predator species such as raptors. Species with laterally placed eyes, such as parrots, have a larger visual field (300o for pigeons) versus frontally directed eyes (150o for barn owls). However, as the visual field increases, binocular vision decreases. In binocular vision, both eyes focus on the same object, and eye movement is coordinated. Monocular vision occurs when only one eye is focused on one object at any particular moment. That type of vision is the norm for our parrots.
The eyeball consists of the small anterior cornea, a variable intermediate region, characterized by scleral ossicles and the posterior sclera. These sclera ossicles provide the rigid shape to the eyes of birds, which is not the way mammal eyes get their shape. Types of eyeballs include:
- Flat eyeball — diurnal birds (active during the daytime) with narrow heads; a short bulbar axis results in a small visual image on the retina and lessened visual acuity.
- Globular eye — diurnal birds with wider heads, including passeriforms, most parrots and birds of prey; a cone-shaped intermediate region results in greater visual acuity
- Tubular eye — nocturnal birds of prey; the intermediate region is relatively elongated.
The shape of the avian retina is relatively flat, meaning that its surface lies near the point of focus for all directions of incident light. The wall of the eyeball consists of:
- Outer fibrous tunic — cornea and sclera
- Middle layer — vascular layer
- Inner layer — nervous (retinal) layer
Fibrous Tunic
The outermost layer, or fibrous tunic, maintains the shape of the eye. The cornea is relatively small, particularly in underwater swimmers. It is strongly curved in tubular and globular eyes. The refractive index between the air and the cornea is relatively larger compared with that between the water and cornea, and is nearly the same underwater.
The sclera in birds is reinforced with a continuous layer of hyaline cartilage, except at the scleral ossicles. These ossicles form a continuous ring of overlapping bones that support and form a base of attachment for the ciliary muscles. Mammals maintain their eye pressure with the fluid that is made internally. Birds also make fluid in their eyes, and this fluid is drained out by the scleral venous sinus, or canal of Schlemm. This sinus is at the sclerocorneal junction or limbus of they eye, and it needs to stay open to drain fluid or it will cause an increase in pressure like in mammals that get glaucoma.
Vascular Tunic
The middle layer of the eye consists of a continuous layer — the vascular tunic. It is composed of the choroid, ciliary body and iris. The choroid tends to be thick, highly vascular and darkly pigmented, and it provides a significant portion of the nutrition to the eye. The tapetum lucidum, a highly reflective surface in many species, has been observed in only a few species of birds — the goatsuckers, which are nocturnal. The choroid continues as the ciliary body and then the iris.
The ciliary body suspends the lens with its processes, the ciliary processes and its fibers, the zonular fibers that encircle the lens. The ciliary processes are pressed firmly against the lens with its muscles that provide accommodation. These muscles are striated in birds, and they are directly attached to the lens capsule. This unique feature allows birds to rapidly adjust their vision as they fly. These muscles are most highly developed in hawks because they require rapid accommodation as they descend in a dive to hit their prey target with great accuracy.
Accommodation is much different in birds compared with mammals. In mammals, the ciliary muscles result in a passive change of the thickness of the lens. Birds, however, use a variety of active mechanisms for accommodation of the lens. These mechanisms include:
- Using the posterior sclerocorneal muscle to force the ciliary body actively against the lens to change its shape
- Using the anterior sclerocorneal muscles to distort the center of the cornea (hawks, owls)
- Having a softer lens with powerful sclerocorneal muscles to force the lens to bulge through the pupil (diving birds). Water reduces vision because there is no longer corneal refraction, which accounts for 20 diopters.
The iris is often dark but may be highly colored. Male cockatoos often have a black iris, while females have a brown one.
The pupil is commonly rounded; pupils with an irregular margin may result from avian leukosis. Pupillary size is regulated by striated, rather than smooth muscles — the sphincter and dilator muscles of the pupil. Movement and size are rapidly changed in birds, but because of willful movement, a bird’s pupils often not respond to light in a pupillary exam because of stress. Birds also have the ability to regulate the quantity of light reaching the retina by migration of pigment in special cells embedded into the retina. With light adaptation, the pigment migrates to shield the receptor cells.
Lens
As indicated, the lens of birds is softer than that of mammals to provide rapid accommodation. Part of this softness results from the lens vesicle, which is fluid-filled. It lies between the annular pad (Ringwulst) and the body of the lens. The annular pad encircles the equator of the lens and is most pronounced in diurnal predators. In primates, the lens filters light below 400 nm, making it impossible to detect ultraviolet light. On the other hand, birds are able to visualize wavelengths down to 350 nm, allowing them to visualize many things we cannot. They are able to discern males from females in what we think are sexually monomorphic species. They can detect ripeness of food items because of this quality and hawks can visualize urine trails of mice. Now that is something very different than mammals and provides them with unique abilities to see things that we cannot see.
Retina
The retina of birds is relatively thickened and does not contain blood vessels, as occurs in mammals. This allows the entire space to pack in more “pixels” to see with. The retina consists of a non-nervous pigmented epithelium and a nervous layer composed of rods, cones, bipolar cells and ganglion cells. The ganglion cells collect to form the optic nerve at the optic or blind spot.
The rods and cones are the receptors in the retina. Rods are sensitive to the intensity of light, so nocturnal birds have mostly rods. In order to increase sensitivity to low amounts of light, several rods synapse with a single bipolar cell and several bipolar cells synapse with a single ganglion cell. However, the sensitivity of the owl’s eye may be due to its ability to gather more light (2½ times brighter than humans). Their ability to hunt in near darkness may result from their sense of hearing.
Cones are responsible for visual acuity and color vision. In diurnal passerines and predators, one cone synapses with a single bipolar cell, which synapses with a single ganglion cell. This one-to-one projection to the brain greatly enhances visual acuity, or the sharpness of detail.
Factors that affect visual acuity include:
- Relatively large eye
- Accuracy of focus on the regions of the retina because of the shape of the eye (ie, tubular)
- Magnifying capacity of the fovea
- Absence of blood vessels
- The visual acuity or one-to-one projection of receptor cells to ganglion cells
- Amount of contrast between an object and its background
The central area of the retina is the place of maximal optical resolution and may have a fovea. This fovea is commonly deeper than that of primates, thereby increasing visual acuity. The fovea may have one of three arrangements:
- A single, round central area in each eye, close to the optic axis. A horizontal central area is present in water birds and those that live in open plains, which allows the eye to fix the horizon at its reference point.
- Two foveate areas — one in the central area and a laterally situated temporal one. This is common in fast-moving birds, which require accurate perception of distance at relatively high speeds.
- A single foveate area temporally placed, as in owls.
Color Vision
There are three visual pigments in birds, with a possible fourth, that are sensitive to near ultraviolet wavelengths. In birds, cones additionally have an oil droplet within them with five different absorbency spectra. The droplets’ function is not entirely known, but they may directly produce color vision or may enhance contrast by acting as intraocular filters. For example, yellow droplets could remove much of the blue from the background, increasing contrast between an object and the blue sky.