Welcome to the world of avian vision! And to one observer’s thoughts about a part of avian anatomy that may, in some bird species, have an additional function to those normally attributed to it. The mysterious nictitating membrane…
LET’S START WITH SOME OBSERVATIONS
1) Most birds keep their head and eyes still and blink with their third eyelid (nictitating membrane) every time they move their head or eye.
2) A bird’s brain is relatively small in comparison to its eye size.
3) A bird can identify insects more quickly than humans can.
4) Studies show that some bird’s retinas have five times more rods and cones than humans.
5) Most birds have eyes either side of their head and see two separate views with very little frontal stereoscopic vision. Owls are one typical exception.
6) Jerome Lettvin’s 1959 paper, ‘What the Frog’s Eye Tells the Frog’s Brain’, describes how a frog can only see its food, small flies for example, as movement due to its retina having a rapid sensitivity decay in some of its rods and cones. He claimed that if a frog was put in a box surrounded by dead flies it would starve to death because no movement would be detected and it would be unable to recognise them as a food source.
7) Human eyes also have some amazing qualities like the rapid decay in sensitivity in bright areas while areas where there is less light remain highly sensitive. This is called ‘Instantaneous Contrast’ and allows us to see detail in both highlight and shadow areas.
8) Our human eyes make two basic types of movement: jerky movements called saccadic movements and smooth continuous movements called pursuit movements. The only time our eyes move smoothly is when they are tracking something. At all other times our eyes move with small jerks.
9) The retina, being neural, is actually a part of our brain.
10) Small birds blink so fast during each head movement that human eyes cannot register it, but a high speed video close-up of a bird’s eye will show the nictitating membrane in action.
AND NOW, SOME QUESTIONS
1) If a bird’s eye is five times better than a human’s how can it process all that visual information with such a small brain?
2) Does a bird only need to see essentials like food, predators and its mate?
3) Why do small birds blink so fast that we can’t see it happen?
4) Why do birds appear to only use jerky ‘saccadic’ eye movements?
5) Why is the nictitating membrane (third eyelid) milky like a cataract?
6) Why do birds only blink whenever they move their heads?
7) Why does their typical prey tend to remain completely still and then move in quick bursts to the next motionless position?
8) If birds evolved from dinosaurs, did dinosaurs also have a nictitating membrane and move in a similar way to birds today, keeping their head and eyes still to help detect movement?
SHARP’S HYPOTHESIS ON BIRD VISION
Taking into account the observations and questions above we might hypothesize that a bird needs to reduce the amount of visual information being sent to its small brain by using the retina to edit what it sees. It may do this by using a vigorous form of instantaneous contrast (retinal decay) to send to its brain only vital information, like the detection of food or predator activity – a bit like the frog.
The nictitating membrane which blinks over the eye to lubricate it may also blur the image during its shift to a new position so that no edge movement is seen and, therefore, the brain would have less visual processing to do. The blink may also help remove any burnt-in after image so the next view would be with a refreshed retina.
THE BEHAVIOUR OF OTHER ANIMALS
As a result of this extremely efficient movement detection in birds’ eyes we can assume that prey in the form of insects and small mammals must have adapted their movements to avoid predator detection in order to survive. Perhaps this is why they stay perfectly still and move only in quick jerks to help avoid detection. Those that did not may have died off leaving the smart ones to be with us today.
Birds like the White Collared Manakin dart from side to side during their mating dance. Would this be so their partner can see them? Also they make a clicking noise which may cause the nictitating membrane to blink and refresh the retina so its mate becomes even more visible.
COMPUTER GENERATED SIMULATION
It is possible to simulate how a bird may see by using a basic video camera with some editing software. Film, from a fixed position, a close-up of a patch of grass for a few seconds (preferably with an ant or a fly walking across it). Then, using a video editing program like Final Cut Pro, copy the sequence and paste it over the same image but out of sync by a delay of a few frames. Turn this film layer composite mode into ‘difference’ and you will find that all static parts of the image are cancelled out revealing only the ant or other moving insects. This technique attempts to replicate retinal decay as described by Dr Jerome Lettvin’s paper about the frog’s eye and, remarkably, it can reveal insects that may not normally be visible to the human eye. However, if the position of the camera were to shift rapidly to a new viewpoint, as a bird does when searching for food, the result would be a very confusing image. By placing a milky filter in front of the lens during this transition (which represents the nictitating membrane blink) the image would blur, thereby removing all movement information and representing the much reduced visual signal that a bird’s brain might receive.
THE NEXT STAGE…AND THIS IS WHERE, MAYBE, YOU CAN HELP
There must be thousands of hours of existing footage taken from fixed camera positions which, if re-examined using this ‘difference’ editing technique, would reveal a vast amount of new information that may not have been noticed at first viewing. If you are interested in creating a project to review the observations the author has made please leave a comment below…
A 9 minute video for ‘The Third Blinking Eyelid’ can now be downloaded free from http://vimeo.com/38353234
All text and images on this site © Jim Sharp 2011