Avian vision

(Left) This article explores aspects of avian vision varying from the placement of eyes on a bird's head, to the eye's general structure and even the types of cells found in the eye.

Vision is of high importance for birds, which have much more acute vision than humans. The vision capabilities of many birds are astonishing - some species of eagle can see small prey animals (such as rabbits) from two miles away! But why do birds have more acute vision than we do and how different really are avian eyes to our own? This article summarises some of the key features of vision which are shared between several species of birds, though even in birds, vision can vary quite significantly between different species.

Structure of the eye

Diagrams to show the structures of a human eye and a bird eye.

Compared to our eyes, avian eyes are proportionally larger compared to the size of their brains, and can be equivalent to 50% of their skull volume, compared to only 5% in humans.

In terms of its general structure, a bird’s eye is quite similar to a human eye. However, a key difference is that avian eyes contain an additional structure known as a pecten oculi (pecten). This is a vascular structure comprised of pigment cells, large blood vessels and many capillaries, which provides nourishment for the retina in the form of nutrients and oxygen. The pecten is attached to the optic disc at the end of the optic nerve. This is important as it means that it can provide nourishment for the retina without compromising vision, as there are no light receptor cells on the optic disc. In comparison, the mammalian retina has a direct blood supply. While this is useful in directly providing oxygen and nutrients, the blood vessels themselves may compromise vision; this may be one reason why birds have more acute vision than humans. In most species, the pecten has many folds in its structure, creating a large surface area, although the number of folds varies between species, depending on the species’ visual requirements (e.g. if the bird is diurnal or nocturnal). The pecten also regulates the pH of the vitreous body, the fluid which fills the space in the eye between the lens and the retina.

Like human eyes, birds' eyes contain a cornea and a lens, which are both important structures. In birds, both the cornea and lens can change shape (in a human eye, only the lens can change shape) and the lens is softer and more flexible than a human lens. This means that, by taking advantage of these features, birds can focus on an object more easily than humans. In an avian eye, there are no suspensory ligaments; instead, there is muscle directly around the cornea and lens.

Avian eyes are not spherical in shape, as human eyes are. Although the shape varies according to the species of bird, avian eyes are flatter at the back than human eyes, so there is a greater area of the retina on which an image can be focused.

Tetrachromatic vision

Like humans, birds have two types of photo-sensitive cells in their retina - rod cells and cone cells. The high density of these receptor cells in their retina enables birds to have more detailed vision than humans. Usually 80% of a bird’s receptor cells are cone cells, however, in owls, nearly all of the receptor cells are rod cells, as rod cells are more light-sensitive so enhance owls’ vision in dim light.

Humans have three types of cone cells, for detecting red, green and blue light, which means that we have trichromatic vision. Birds, however, have an extra type of cone receptor which contains visual pigments sensitive to ultraviolet (UV) light, which enables them to detect light in the near-ultraviolet range of wavelengths (300-400 nanometres). Although in some birds UV light is absorbed by the cornea or lens before it reaches the retina so it cannot be detected, other species have full sensitivity to UV light, including passerines, gulls, ostriches and parrots. Research suggests that these species may use this sensitivity to detect fruits in foliage which reflect UV light, or it may add a depth to plumage patterns which we can’t see.

Foveae

Also present in both bird and human eyes is a fovea, which is a region of the retina with a higher density of rod and cone cells than the rest of the retina, leading to sharper vision there. Around 50% of bird species, though, are bifoveate, meaning that they have two foveae in each eye. The deep fovea, centrally located in the retina, enhances a bird’s monocular vision of a distant target, meaning that the bird’s eyes can move independently. The shallow fovea, in the lateral region of the retina, helps with binocular vision (as in human vision). Having two foveae helps a bird to better perceive the distance and speed of its prey.

Depending on the position of the eyes within a bird’s skull, birds have varying fields of view. Birds such as a Robin have excellent lateral vision but minimal forward vision, meaning that they are unable to see the tip of their beak. In contrast, birds such as a Woodcock, which have eyes placed high up on the sides of their head, are able to see better than a Robin around and behind, but do not have good forward vision.

This shows the placement of the eyes on the head of a woodcock and robin.

A third eyelid

In addition to the eyelids, most species of bird have a ‘third eye’ called a nictitating membrane which also covers the eye. It is a translucent membrane made up of thin sheets of tissue which is used to moisten or clean the cornea, quickly lubricating the eye as it moves across it (as opposed to the usual up and down movement of the eyelids). It is especially useful for birds flying in windy conditions which could dry their eyes out. Also, it acts as a form of protection for the eye, and may sometimes be closed when a bird is feeding its young, flying between branches or capturing prey, when there is a risk of the eye being damaged.

Although there are similarities between avian and human vision, bird eyes have many more adaptations which enhance their vision compared to humans. To summarise, the table below highlights the key differences between them.

Human vision

Eye is approximately spherical - no pecten, suspensory ligaments

Trichromatic vision (3 types of cone cells)

No nictitating membrane

Avian vision

Shape of eye varies according to species - more oval than spherical - contains pecten but no suspensory ligaments

Tetrachromatic vision (4 types of cone cells)

Nictitating membrane + eyelids