Farm life

When it comes to colour vision, chickens have bested humans in many ways

Chickens have a large field of vision

Remarkable Chicken Vision

POULTRY HAVE EXCELLENT VISION, EXCEEDING OUR OWN IN COLOUR SENSITIVITY AND RATE OF CAPTURE. CHICKENS RELY ON THIS SENSE FOR FORAGING, AVOIDING PREDATORS AND CHOOSING MATES.

By Tamsin Cooper

organ, highly tuned for detecting colour and movement. Chicken eyesight is arguably better than ours in some respects. It is important for keepers to recognize their abilities and limitations, so that we can provide an interesting and comfortable environment for our flocks.

More Colours in the Rainbow

First of all, their colour sensitivity is amazing! Unlike mammals, birds never went through a nocturnal period during their evolution. To avoid predators, mammals’ eyesight developed to allow foraging at night, while sensitivity to ultra-violet and red light was lost. Some apes, including humans, re-evolved sensitivity to red when we switched to being active during the day. Birds descended directly from dinosaurs and retained their highly sensitive daytime vision. Chickens see ultra-violet as well as all the colours we see. UV vision helps them to detect movement and avoid predation. It also enables them to see patterns in plumage and vegetation that we are not aware of. Studies show that hens spend more time checking out males and mate more often when UV light is available. Chickens are more sensitive to red and blue than we are. This helps them to pick out important features against the green of field and forest. Red is typical of nutritious food sources and chickens are attracted to this colour. However, they know to avoid red insects, which are normally poisonous, and they readily learn new associations of colour with favourite foods. This means that they will get used to food of a certain shape and colour, and may refuse a new type of feed until they learn that it is palatable. Other senses come into play when choosing food, such as texture, smell and taste, so any changes in these might put chickens off. Light is also detected directly in the pineal gland of the brain after passing through bone and tissue. Here it is used to regulate daily and seasonal rhythms.

Complex Eye Structure

The retina is highly structured with four types of colour-sensitive cone cell that respond to wavelengths between ultraviolet and red (350–780 nm). For Tamsin Cooper is a smallholder and writer with a keen interest in animal behaviour and welfare

The avian eye is an amazingly complex

www.goatwriter.com comparison, humans have three types of fluorescent lighting during the day, to cone that detect light of 380–740 nm. In ensure healthy development. addition, chickens have double cones, which aid in detection of movement. Oil Visual Limitations droplets within birds’ cones act as filters, Despite their amazing vision, it is enhancing contrast and colour detection. important to understand what chickens Cones are concentrated in certain areas to cannot see. As animals that evolved for intensify detail, in the same way as our daytime living, they see poorly in the dark. fovea gives maximum clarity in the central This is why they quickly go to roost at area of the human eye. In chickens, these sunset and do not stir until dawn. A hen sensitive areas extend upwards and unable to find her way back to the coop towards the beak, to allow more detailed will struggle in dim light that humans can perception of danger overhead and adjust to. Their pupils are not able to potential food sources on the ground. adjust as much as ours do and they have Independent Control fewer rods (the light-sensitive cells in Chickens have an eye on each side of the he retina). head, giving them a wide range of vision of Detail is quickly lost as light fails. Their about 330°, with an image overlap of 26° complex cone structure does not match in front of the head. While images from the acuity of our fovea, although their each eye are processed independently, the overall clarity of vision is probably as good overlap provides fine during daylight. To control for moving gain a complete picture forward and Either eye can fix on a of any object, a chicken manipulating objects nearby object, while the must move her head with the beak. The monocular regions at the side give good peripheral other remains focused at distance around to expose it to the various specialised areas of her retina. vision to scan for Detailed social predators. Just a narrow recognition appears to area behind the head is not seen. be effective only at distances of less than However, chickens move their heads 20 cm, although chickens have the around to reduce this blind spot, making impressive ability to memorise at least it virtually impossible to creep up behind them unnoticed. 60 individuals. This near all-round vision is enhanced by chickens’ ability to focus their eyes independently. Either eye can fix on a nearby object, while the other remains focused at distance. Moreover, each eye has a lower field that focuses at a nearer distance than the upper field. Similarly, each pupil contracts independently to adjust to different light conditions. As chicken eyeballs are large compared to the head, there is little room for the eye to move in its socket. The image needs at least 20 ms to register on the retina, resulting in chickens’ characteristic head movements. You will notice how their heads bob as they walk, the head staying fixed as long as possible as the legs move forward, then rapidly moving forward to catch up with the body. This helps to Built for Speed register a clear image. If you pick up a hen You may have noticed how quickly and gently move her body around with chickens move – they can even catch flies! small movements, you will see how her They change focus by stretching the lens neck compensates to keep her head and bulging the cornea, resulting in rapid stock still. focus shifts. Their pupils react four times The precious eyes are protected by the faster than ours do. In addition, they nictitating membrane. This transparent perceive changes much more rapidly than we do. They even see the flicker of indoor light bulbs and computer screens. In most conditions, chickens are aware of flicker up to about 95 Hz. If you are raising third eye flicks in briefly from the inside corner of the eye to protect and moisturise the eyeball. As it is for us humans, vision is the most important sense for chickens. chicks indoors, make sure they receive Source: Nicol, C.J., 2015. The bright daylight, or at least warm Behavioural Biology of Chickens, CABI