Most owls are nocturnal or crepuscular, which means they are active at night or during the twilight periods of dusk and dawn. Because much of their prey such as mice, voles, and other rodents are also active at night, owls need to be able to see in the dark so they can hunt their prey.
Owls’ eyes have adapted in numerous ways to allow them to see in the dark and dim light, and they also have evolved other characteristics to enhance their vision.
Owls’ eyes are similar to ours in that light enters the pupil through the cornea and passes through the lens. The lens and cornea work together to focus the light on the retina situated at the back of the eye. Cells called photoreceptors turn the light into electrical signals which travel through the optic nerve to the brain, which makes sense of the information it receives and turns it into an image.
There are two types of photoreceptors in the retina – cone-shaped cells and rod-shaped cells. Retinal cones function at high light levels (photopic vision) and are responsible for colour vision and spatial acuteness, whereas retinal rods are responsible for vision at low light levels (scotopic vision), and only process in black and white.
In an owl’s eye, the retina has a large number of closely packed retinal rods. They have about 30 times more rods than cones which means that although they can’t see colour very well, they have excellent night-time vision.
Because owls have such good night vision, it has been suggested that they are unable to see during the day. But this is a common misconception. In bright light their pupils will adjust to allow less light in, unlike some other nocturnal animals that can only see in the dark. Owls will also partially close their eyes in daylight to block out the light. This can make them look half asleep when they are actually wide awake and alert.
Behind the retina in an owl’s eye is a layer of tissue called the tapetum lucidum, a retroreflector, that acts like a mirror and reflects visible light back through the retina. This gives the owl a second chance at collecting the light available and sending it to the photoreceptors, which helps them see better in the dark.
When light enters the eye of an animal with a tapetum lucidum the pupil appears to glow, an effect known as eyeshine. Humans don’t have a tapetum lucidum, but you can see something similar happening with flash photography when it causes ‘red-eye’. In this case the light has been reflected off the fundus of the eye, the interior surface opposite the lens.
To allow as much light to enter the pupil as possible even in dim conditions, an owl’s eyes are huge with enlarged corneas and lenses, and can account for up to 5% of its body weight. Compare this with a human eye which is amongst the smallest of our organs, and with both accounting for just 0.02% of our body mass.
In fact, an owl’s eyes are so large that they aren’t spherical at all, but are elongated tubes that are held in place by fixed bony structures in the skull called sclerotic rings. However, this means that they can’t roll or turn their eyes to follow an object and can only look straight ahead.
To see what’s around it, an owl must rotate its neck by up to 270°. It can do this without injuring itself because an owl’s neck is only connected by one socket pivot rather than two, and it has holes in the bones of its neck that act as cushions to protect the arteries.
Because their eyes are situated on the front of their head instead of the sides, owls have what is known as binocular vision. This means owls can see an object with both eyes at the same time in 3 dimensions – height, width, and an increased depth perception.
An owl’s field of view is about 110°, with about 70° of that binocular vision. A bird with eyes on the side of its head has a much larger field of view, anything up to 300°, but has much narrower binocular vision, sometimes as little as 10°.
One drawback of such large eyes is that owls are very long-sighted and can’t focus on nearby objects. To compensate, they have a group of short, bristle-like feathers around the beak called crines. Crines are sensitive to the touch and help an owl locate dead prey once it is caught. These are the first feathers an owl develops so it can find food brought to it by their parents. Crines also help remove dried blood away from an owl’s mouth to prevent it from getting sick.
Like all birds of prey, owls have 3 eyelids. The upper one closes completely downwards when the owl blinks while the lower closes upwards when the owl goes to sleep. The third eyelid is called the nictitating membrane and moves horizontally across the eye from the inner corner to the outer corner. It helps protect and moisten the eye but because it’s translucent owls can still see when it’s closed. The membrane also helps protect parents’ eyes from chicks when they are feeding them.
Not all owls are nocturnal. Some are diurnal which means they are active during the day. It is often claimed that you can tell the hunting behaviour of an owl from the colour of its eyes. Owls with dark brown or black eyes are nocturnal, owls with orange eyes are crepuscular, and eyes with yellow eyes are diurnal.
However, although there is some correlation between the colour of an owl’s eyes and the time of day it is most active there are some notable exceptions, such as the barn owl which has black eyes but is crepuscular and diurnal, or the little owl which has yellow eyes but is mainly nocturnal.
A paper published in 2020 found that the ancestor of the family Strigidae or true owls was more likely to have had bright irises, while the ancestor of the family Tytonidae or barn owls was more likely to have had dark irises.
The results showed support for the coevolution of iris colour and nocturnal activity although does not show why they coevolved, as there is little support for the commonly held idea that dark irises in owls may be a way for them to camouflage themselves at night.
This article was originally published by Birdspot.co.uk. Read the original article here..
