Parrot's Eye

Parrot's eye

Basic Anatomy and Physiology – The Bird’s eye

The eye is the predominant sensory organ in birds. Watching your parrot closely will make you realize how well they see and how much they rely on their visual sense. While the basic plan of the bird’s eye is the same as in mammals, birds have a few unique features that sets their vision apart from ours. 

Facial shape and eye shape

Parrots are narrow-faced, day-active birds, so their eye bulb is flat and shallow and set at the side of the head. This leads our birds to see only with one eye at a time (monocular field of vision). So a bird actually looks at you full-on when he/she turns the side of the head towards you. The field of vision where a parrot sees “stereo” with both eyes is only about 6-10°. In return, it gives them a field of vision close to 360°, so they can see most of what happens in front, behind, above and below them. In addition, parrots can move their eye bulbs in the sockets (not all birds can), which allows them to look around without moving as a protection from predators.

Scleral Ring

Unique to birds (and some reptiles), the sclera ring is a bone structure around the lens. It protects the eye bulb (especially in birds with a shallow eye socket) and helps with the accommodation of the lens to focus on objects.

Sclera

Outer eye membrane. Gives the eye structure and keeps the inner eye pressure constant.

Uvea

Middle eye membrane, consists of the chorioidea, the corpus ciliare and the iris. Supplies the eye with nutrients (chorioidea) and accommodates the lens for sharp images (corpus ciliare). The Iris (the part of the eye which is blue, green or brown in us humans) is the “shutter” of the pupil. In birds, the muscles attached to the iris are striated, so the bird can actually control opening and closing the pupil. This allows much faster adaptation to bright and dark (e.g. during flight). 

Pecten oculi

This structure at the back of the eye where the visual nerve enters is unique to birds and some reptiles. It can have a folded, fan-like or conical structure. It’s function remain unclear, but it is suspected that has a nutritional function as well as regulating pressure and temperature inside the eye.

Retina

Inner eye layer. This layer is the one that contains the nerve cells for vision. The retina in birds is much thicker than that of mammals, which reflects their excellent vision. It contains two types of nerve cells – cones, which are responsible for daytime and color vision and rods, which are responsible for night vision. The ratio of cones vs rods depends largely on what time of the day the bird is active. In day-active birds, the number and density of cones is high and one efferent nerve cell is only connected to a few cones, which increases the sharpness of the image.

Another special feature of avian cone cells is that they contain droplets of colored oil, which function as filters to increase the width of the color spectrum. While humans have rods that are most sensitive in the areas of red (560nm wavelength), green (530nm) and blue (430nm), birds have 4 or 5 different types, which causes them to see between red (680nm wavelength) and ultraviolet (320nm). Vision in the ultraviolet spectrum is important for e.g. seeing UV reflections off the cuticle of berries, which indicates if they are ripe or not. It is also said that UV light plays an important role in the birds’ circadian rhythm and reproduction, so it is important that your bird gets sufficient UV light, ideally by exposing him/her to natural sunlight. If you have no means to do that, consider artificial lighting containing UV, such as true-light bulbs or compact fluorescent bulbs.

Performance of the bird eye

The bird’s eye is a very specialized organ and thus well adapted to the bird’s activity (active during day / night, underwater vision etc).

Accommodation of the lens: the accommodation of the lens allows the birds to focus close and far away. While humans have a capacity of accommodation of about 10 diopters (D) (and dogs have about 3D), most day-active birds have about 30-50D range of accommodation. At rest, birds focus on things far away from them.

Sharpness of vision: birds and humans have about the same sharpness of vision in the area with the highest resolution. However, while humans have a very small central area, day-active birds have a much higher density of cones and a more homogenous distribution on the retina, which leads to a higher resolution overall. Additionally, the shape of the bird’s eye causes a field of vision that’s evenly sharp and colorful in all directions, and not as focused as us humans. For example, a vulture will be able to see a mouse from 2km away.

Detection of movement: birds see images at a much higher frequency than us humans and thus detect movement much better. This is important for spotting predators (or for raptors spotting mice). E.g. most parrots can see at about 150-200 frames per second (fps), so when they watch television (about 24 fps for normal TV and 50-60 fps for HDTV), it looks like a slide show to them. Birds can also perceive very slow movements, such as the movement of the sun.

This should be taken into account with lighting of the area your parrot is in. Most regular neon tube lights have a frequency of 120Hz, which probably looks to your bird as if someone is constantly turning the light switch on and off. So in order to avoid behavioral issues you should make sure to at least use a 400Hz tube.