Pet keepers have long known that birds react to our own eyes, seeming to be very aware of when we are and are not looking at them. Researchers at the University of Bristol have now confirmed that birds modify their behavior in accordance with observers’ eye movements (please see “Further Reading” below).
Vision is the most important of the senses for most bird species…the optic lobe of a bird’s brain, and the size of the eyes in relation to the head, are comparatively greater in birds than in mammals. Today I’d like to pass along some interesting facts on avian eyes.
Location of the Eyes
The eyes of predatory birds such as eagles are set close together at the front of the head, allowing for binocular vision (both eyes focus on the same subject) as in people.
Birds that are hunted by other animals usually have eyes that are set well-off to the sides of the head. This allows for a greater range of vision and helps them to spot danger. The ring-necked dove’s field of vision is 300 degrees, and by a slight movement of the head reaches 360 degrees.
Birds focus their vision by muscular action which changes the shape of the lens and/or cornea. The avian eye lens is softer than that of mammals, allowing for quicker focusing.
Visual clarity is also affected by the angle of light as it passes through the eye’s cornea (as well I know – my corneas are pointed, and I’m virtually blind without a covering lens to push them back into shape!). Cornea angle changes allow penguins to see well under-water, and diving birds such as cormorants to switch quickly from water-based to air-based vision.
Color Vision and Acuity
Structures known as cones and rods affect acuity (clarity) and color vision. Birds see objects located at the very edge of their field of vision quite clearly – we see only “glimpses” and must turn and focus. Most see at least 3-4 colors, but more research is needed.
The retinas of some birds have depressed areas, known as foveas, which are supplied with extra cones, allowing for particularly acute vision. Foveas enable hawks to assess speed and distance while swooping down on prey. Kingfishers, which dive on fish from above, have two foveas in each eye – one for air acuity and one to help them see when submerged.
Rods are light sensitive, and are particularly abundant in the eyes of nightjars, owls and other nocturnal birds.
The eye’s pupil regulates the amount of light that reaches the retina. Birds change the size of their pupils with highly specialized muscles. The pupils of bird eyes are thicker than those possessed by most mammals, allowing for greater light penetration and sharper vision.
The Third Eyelid
Birds possess a clear, third eyelid, or nictitating membrane. This membrane prevents the cornea from drying out during flight and offers protection underwater.
Please see my article European Starlings can Determine When People are Watching for some interesting research and a personal story about the visual powers of an owl that liked to feed people.