Through the eyes of a chicken...

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Sunday, March 18th was an absolutely beautiful day in New Hampshire.  The temperature hovered around the mid 70’s, the sun was shining in all directions, and there wasn’t a cloud in the sky!  I could see cars driving by with kayaks strapped to their roves, people enjoying a mid-day walk, and it seemed like just about everybody was outside inventing outdoor home-improvement projects.  So, the good son that I am, (or maybe it was just because I couldn’t bear to waste the sunshine) I decided to help my parents with their little house projects…on their farm.  Yes, for the day I decided to be farmer Bart.  And as I was scooping out chicken poop while the wonderful sent of nitrogen rich air entered my lungs, I noticed a hen peck at a seemingly absent morsel and devour it whole.  I wasn’t about to figure out what that morsel actually was--in fact it was probably better that I didn’t--but I thought to myself how could she have possibly seen that?  That’s when I decided to learn more about a chicken’s visual system!  I came across some interesting facts. 

                You may already know how the human eye works, but humor me while I go into detail.  The human eye contains cells called rods and cones that receive light and are part of a complex pathway that is responsible for delivering a visual message to the brain.  Simply put, rods are designed for night-vision and detecting motion, while cones are designed for daytime vision and discriminating fine detail.  Another benefit of cones is that they are used as part of our color vision.  Our eyes have three different cones that are the most sensitive to three colors; red, blue, and green.  You may ask, “Why can we see many different colors when we only have three light absorbing cells?” (or something to that effect).   Well, our eyes are structured in such a way that we have the ability to turn on and off specific color channels.  These channels will combine to form other colors.  Think of it as if our eyes were painting a picture using only three key colors. 

A picture of rods and cones in the human retina magnified 

2,000x their original size.  The smaller cells are cones.

                                                               http://www.sciencephoto.com/media/308755/enlarge

                 Now let’s talk about the chicken.  Instead of 3 different cones, chickens have 5 different cones in their eyes!  One cone is actually a violet/ultra violet light absorbing cone.  The cones are arranged in such a complex pattern around the eye that color and fine-detail discrimination are extremely easy for the chicken.  Additionally, human eyes have a spot in the retina the size of a pin-point where the largest numbers of cones are clumped together.  This spot is called the fovea, and is the only area of our eye we use for 20/20 vision.  In the chicken it is a strip, instead of a spot, allowing for more cones to be bunched together and more surface area for a greater amount of light absorption.  In fact, some birds actually have 2 areas where this occurs!  The arrangement of these fine-detail, color discriminating, daytime vision cells leaves very little area for the night-vision rods to dwell in.  This is why chickens have a very difficult time seeing at night.  They usually run to their coops before the sun goes down.  This made me wonder if chickens have a hard time seeing motion due to the lack of rods in their eyes.   But, as I read on I came across a section that stated the 5^th cone that was discovered in a chickens eyes, called the “double-cone”, may be used as a motion detector in place of the low concentration of rods.  To summarize, chickens, and birds in general for that matter, have better color vision and sharper detailed vision than us humans.  So if you see a chicken, just remember its staring back at you!