Chicken Vision

You should be able to file this entry into the “stuff I didn’t know” category.  I have had some experience with chicken vision, and was aware of some of the basic ideas of the “weirdness” that bird sight entails.  As I started writing this, I wanted to make sure I got my facts right (ish), so looked up a few things, and they led to a few more, and half my evening got lost in obscure eye facts that will never be clinically useful to me, but will be GREAT fodder at the next cocktail party.

How do chickens see?  What do chickens see?  Why do they bob their heads around like that?  Why do they look at you sideways?  Can they see at night?  How much can blind chickens see? 

Chickens see the same way we do….light comes in through the cornea and iris,

Chicken eye....about 25 times as large as a human eye...as a percentage of head size

then stimulates nerve endings in the retina at the back of the eyeball.  A major difference, however is that chickens have tetra-chromatic vision, while we have tri-chromatic.  In english, chickens have 4 wavelengths they are sensitive to, while we see 3 (red, green and blue).  The chicken eye sees red, green and blue as well, but they are also sensitive to ultraviolet light.  This seems kinda interesting at first glance, but the implications are actually staggering.

The fact that chickens see an extra sector of the light spectrum means that EVERYTHING they see looks different from what we see.  Their concept of the

The 4 peaks of sensitivities of the chicken eye.....they see UV light (grey line), as well as all the colours we see

green colour of grass is as different as our comparison of aquamarine and the colour of grass.  We have no concept or description for how much UV is reflected from any substance.  There is evidence that birds can find direction by looking at the sky and seeing the gradation of UV, and knowing which way is north as easily as you or I looking at a grey-scale drawing and knowing which side is closer to white.  It also means that we have a really hard time understanding what they are seeing.  In the following picture, there is a cockatiel.  Males and female cockatiels look the same to us, but if you

Left: bird and egg the way RGB eyes see them….Center: UV reflection of the same bird and egg….Right: What a chicken sees….

 look at the UV contribution, you see something else.  The picture on the left is human sight…..the picture in the middle is UV spectrum only, and the right hand picture is a rendering that approximates how another bird would sense that bird. 

 

Now….chickens have a disability when compared to us….their night vision is poor. This is a big part of the reason that chickens need protection at night from predators.  The retina in mammals is made up of rods and cones…..rods to see at night, and cones to see color.  Chickens have very few cones, and they are not especially sensitive.  This difference between rod to cone ratio, and the light sensitivities of cones in birds vs mammals is explained because mammals all but disappeared from evolution long ago, and the only types of mammals that survived  were nocturnal and insect eaters.  Mammals that survived this evolutionary bottleneck re-developed colour vision after millions of years, but since we evolved our cones from a different starting point than birds (they evolved from dinosaurs, and never spent millennia as nocturnal creatures), we developed our colour vision a little differently.  It’s another case of convergent evolution….kinda like whales and dolphins evolving to look like fish, because that’s the body type that works best in the water.

Bird’s colour vision is also different from ours because they have coloured filters mixed in with their nerve cells……little coloured drops of oil filter out different wavelengths, and act similarly to wearing yellow goggles when skiing

This is what you look like to a chicken....or at least a good guess

on a bright day….the contrast is enhanced.  Now imagine wearing yellow and blue and red goggles all at the same time…..it increases contrast and brightness and sensitivity, all at once, and we mammals can’t even imagine what it might look like. 

Chickens also have much better motion sensing ability than we do.  Not as good as hawks, but better than us….again because of a structure called a double cone in the retina.  This is important if you use flourescent lights in your coop.  Flourescent lights flicker on and off at a rate above what we can see….you notice it on old flourescent tubes that are dying….the flicker rate slows down and we can see it.  It is exceptionally annoying.  Birds can see the flicker in many flourescent lights, especially dimmable ones that are at lower intensity.  It would be like being in a dance club with strobe lights on…..all the time….it drives them nuts…literally.  On objects sitting still, chickens may not have as much acuity as we do, however.  This explains why hens are as “spooky” as they are when somebody makes a sudden movement, and why one bird jumping from something can cause the entire flock to take wing, even if they didn’t see the offending stimulus. 

Birds and mammals have a structure called a fovea in their retinas too…..its a small pit that, because of its shape, acts as an image enlarger.  You can see yours in action by looking at something out of the corner of your eye, then looking at it directly in front of you…..its way clearer in front of you, and why you look slightly down at anything you are concentrating on (the fovea is a little above the middle of the retina).  Chickens have 2 foveas (fovei?), and they act a little differently.  One is for distant vision, and one is for in close….think of built-in bifocals.  The funny thing is that the up close one is oval, and sideways….thats why, when you approach a bird, once you get to the focal distance of about 2-4 feet, birds will often bob their heads, and tilt their heads somewhat sideways to get the image better lined up on the second fovea.  Birds actually can’t reliably recognize flock mates until they are within about 24 inches.

Finally, blind birds can see light.  Birds reproductive cycles are controlled by their pineal gland, which is located in the middle of the bird’s forehead, just under the skull.  The skull is thin enough that reasonably bright light penetrates it and will still stimulate the hormone cascade that begins lay.  Even blind chickens can “see” spring coming.

This post is way too long already, so I will cut it off here….just remember, even if you and your chickens can see eye-to-eye, you still won’t see what they see…..keep it in mind when you try to figure out why they do what they do.

Mike the Chicken Vet

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