We’ve recently welcomed 3 hens who each have an eye injury. Athos’ right eye was cloudy, Porthos’ left cornea and eyelid were damaged, and Aramis’ left eye was bulging out. Our avian vet found that Athos and Porthos’ wounds were clean and scarred, and while it impacts their vision, there was nothing that can be done. However, Aramis had glaucoma due to trauma and her eye needed to be removed. The surgery went well, and after a week of recovery, she is back with her rescue friends. Trauma like this is unfortunately common in egg farms as chickens are confined in a very crowded environment with very little to do, except pecking at each other! Over the last couple of weeks, I read a lot about vision in chickens and wanted to share it with you.
Chickens are endlessly fascinating and their vision is especially cool.
First, a chicken’s eyes are about 10% the entire size of their head. Birds in general have large eyes compared to most other animals. As a percentage of head size, their eye is about 25 times as large as a human eye. Their eyes are located on the sides of their heads allowing them to see 300 degrees around. With our eyes in front, we can only see about 180 degrees.
Like humans, chicken eyes contain photoreceptors called rods and cones. Rods see shades of gray at night, and cones see colors during the day. Chickens have very few rods which explains their terrible night vision. Humans have three types of cones to see red, green, and blue. Chickens however have four types that allow them to see red, blue, green, and ultraviolet light. This really gives them superpowers that are hard for us to comprehend. Many insects and seeds reflect UV light, but soil and grass don’t. Unlike us, chickens can see these UV reflections, and when they scratch and peck at the ground, it is very targeted to the shiny specks ready to be eaten.
Chickens also have much better motion sensing ability than we do thanks to double cones in their retina. The cones help them hunt by detecting the movement of bugs and worms, as well as give them an early warning of incoming predators. This extra sensitivity makes them irritable by the flickering of fluorescent lights which we can’t notice. Therefore, we should never use this type of lighting in their environment.
Chickens can use each eye independently to focus on different tasks. The right eye is used for activities requiring recognition, while the left is used for activities requiring depth perception. So, a chicken can look for food with the right eye while watching for hawks with the left. Next time you are with your chickens, pay attention to which eye they use to look at you.
Another chicken superpower is that even blind birds can sense light. They have a pineal gland in the middle of their forehead, just under their skull, that is sensitive to light. This gland controls the hormones that modulate sleep patterns and their reproductive cycle.
To protect their powerful eyes, they have a third eyelid called the nictitating membrane. It slides horizontally over the eye to protect it from dust and debris. It’s also transparent, so the chicken can still watch out for danger.
Aren’t chickens just amazing?