Avian Eye – the High Technology of Nature

Note: This is special blog post about vision.

VISION – Introduction

Vision is the most important sense for any animal, and especially for birds.
Good eyesight is essential for safe flight, and this group has a number of adaptations which give visual acuity superior to that of other vertebrate groups.

The avian eye resembles that of a reptile, with ciliary muscles that can change the shape of the lens rapidly and to a greater extent than in the mammals.
Birds have the largest eyes relative to their size in the animal kingdom.

Image Source: https://www.flickr.com/photos/gallry/

In addition to the two eyelids usually found in vertebrates, it is protected by a third transparent movable membrane.
The eye’s internal anatomy is similar to that of other vertebrates, but has a structure, the pecten oculi, unique to birds.

Some bird groups have specific modifications to their visual system linked to their way of life.
Birds of prey have a very high density of receptors and other adaptations that maximise visual acuity.
The placement of their eyes gives them good binocular vision enabling accurate judgement of distances.
Nocturnal species have tubular eyes, low numbers of colour detectors, but a high density of rod cells which function well in poor light.
Terns, gulls and albatrosses are amongst the seabirds which have red or yellow oil droplets in the colour receptors to improve distance vision especially in hazy conditions.

Photo by A. Sokolowski

Are eyes of birds superior to human eyes?

The retina is a relatively smooth curved multi-layered structure containing the photosensitive rod and cone cells with the associated neurons and blood vessels.
The density of the photoreceptors is critical in determining the maximum attainable visual acuity. Humans have about 200,000 receptors per mm²  ( Average peak cone density near the fovea is 164,000±24,000 cones/mm² ) but the house sparrow has 400,000 and the common buzzard 1,000,000.  The result is better visual resolution.
For example, an American kestrel can see a 2–mm insect from the top of an 18–m tree. The photoreceptors are not all individually connected to the optic nerve, and the ratio of nerve ganglia to receptors is important in determining resolution. This is very high for birds; the white wagtail has 100,000 ganglion cells to 120,000 photoreceptors.

The human retina consists of more than 100 million receptor cells. They are adapted to differen wavelengths of light (red, blue, green), motion, and darkness or dark areas. Additionally, some are sensitive to “light on” and others to “light off”. Similar to a camera with 10-15 different films. A true masterpiece of evolution!

Birds can resolve rapid movements better than humans, for whom flickering at a rate greater than 50 light pulse cycles per second appears as continuous movement.
Humans cannot therefore distinguish individual flashes of a fluorescent light bulb oscillating at 60 light pulse cycles per second, but budgerigars and chickens have flicker or light pulse cycles per second thresholds of more than 100 light pulse cycles per second. A Cooper’s hawk can pursue agile prey through woodland and avoid branches and other objects at high speed; to humans such a chase would appear as a blur.

Perception of fast moving objects: bird vs human eye

The Eye of a Bird

Image Credit: https://www.flickr.com/photos/hugobia/

Some aspects of avian eye anatomy

The eye of a bird most closely resembles that of the reptiles.
Unlike the mammalian eye, it is not spherical, and the flatter shape enables more of its visual field to be in focus.
A circle of bony plates, the sclerotic ring, surrounds the eye and holds it rigid, but an improvement over the reptilian eye, also found in mammals, is that the lens is pushed further forward, increasing the size of the image on the retina.

Most birds cannot move their eyes, although there are exceptions, such as the great cormorant.
Birds with eyes on the sides of their heads have a wide visual field, useful for detecting predators, while those with eyes on the front of their heads, such as owls, have binocular vision and can estimate distances when hunting.
The American woodcock probably has the largest visual field of any bird, 360° in the horizontal plane, and 180° in the vertical plane.

The eyelids of a bird are not used in blinking. Instead the eye is lubricated by the nictitating membrane, a third concealed eyelid that sweeps horizontally across the eye like a windscreen wiper. The nictitating membrane also covers the eye and acts as a contact lens in many aquatic birds when they are under water.
When sleeping, the lower eyelid rises to cover the eye in most birds, with the exception of the horned owls where the upper eyelid is mobile. The eye is also cleaned by tear secretions from the lachrymal gland and protected by an oily substance from the Harderian glands which coats the cornea and prevents dryness.

The eye of a bird is larger compared to the size of the animal than for any other group of animals, although much of it is concealed in its skull. The ostrich has the largest eye of any land vertebrate, with an axial length of 50 mm (2 in), twice that of the human eye.

Photo by A. Sokolowski

Bird eye size is broadly related to body mass. A study of five orders (parrots, pigeons, petrels, raptors and owls) showed that eye mass is proportional to body mass, but as expected from their habits and visual ecology, raptors and owls have relatively large eyes for their body mass.

Behavioural studies show that many avian species focus on distant objects preferentially with their lateral and monocular field of vision, and birds will orientate themselves sideways to maximise visual resolution. For a pigeon, resolution is twice as good with sideways monocular vision than forward binocular vision, whereas for humans the converse is true.

The performance of the eye in low light levels depends on the distance between the lens and the retina, and small birds are effectively forced to be diurnal because their eyes are not large enough to give adequate night vision. Although many species migrate at night, they often collide with even brightly lit objects like lighthouses or oil platforms. Birds of prey are diurnal because, although their eyes are large, they are optimised to give maximum spatial resolution rather than light gathering, so they also do not function well in poor light.^[10] Many birds have an asymmetry in the eye’s structure which enables them to keep the horizon and a significant part of the ground in focus simultaneously. The cost of this adaptation is that they have myopia in the lower part of their visual field.

Nocturnal birds have eyes optimised for visual sensitivity, with large corneas relative to the eye’s length, whereas diurnal birds have longer eyes relative to the corneal diameter to give greater visual acuity. Information about the activities of extinct species can be deduced from measurements of the sclerotic ring and orbit depth. For the latter measurement to be made, the fossil must have retained its three-dimensional shape, so activity pattern cannot be determined with confidence from flattened specimens like Archaeopteryx, which has a complete sclerotic ring but no orbit depth measurement.

The main structures of the bird eye are similar to those of other vertebrates.

The retina is a relatively smooth curved multi-layered structure containing the photosensitive rod and cone cells with the associated neurons and blood vessels. The density of the photoreceptors is critical in determining the maximum attainable visual acuity.

Humans have about 200,000 receptors per mm², but the house sparrow has 400,000 and the common buzzard 1,000,000.

The photoreceptors are not all individually connected to the optic nerve, and the ratio of nerve ganglia to receptors is important in determining resolution. This is very high for birds; the white wagtail has 100,000 ganglion cells to 120,000 photoreceptors.

Rods are more sensitive to light, but give no colour information, whereas the less sensitive cones enable colour vision.
In diurnal birds, 80% of the receptors may be cones (90% in some swifts) whereas nocturnal owls have almost all rods. As with other vertebrates except placental mammals, some of the cones may be double cones. These can amount to 50% of all cones in some species.

Towards the centre of the retina is the fovea (or the less specialised, area centralis) which has a greater density of receptors and is the area of greatest forward visual acuity, i.e. sharpest, clearest detection of objects.
In 54% of birds, including birds of prey, kingfishers, hummingbirds and swallows, there is second fovea for enhanced sideways viewing. The optic nerve is a bundle of nerve fibres which carry messages from the eye to the relevant parts of the brain and vice versa. Like mammals, birds have a small blind spot without photoreceptors at the optic disc, under which the optic nerve and blood vessels join the eye.

Hyperuniformity 
Joe Corbo stared into the eye of a chicken and saw something astonishing. The color-sensitive cone cells that carpeted the retina (detached from the fowl, and mounted under a microscope) appeared as polka dots of five different colors and sizes. But Corbo observed that, unlike the randomly dispersed cones in human eyes, or the neat rows of cones in the eyes of many fish, the chicken’s cones had a haphazard and yet remarkably uniform distribution. The dots’ locations followed no discernible rule, and yet dots never appeared too close together or too far apart. Each of the five interspersed sets of cones, and all of them together, exhibited this same arresting mix of randomness and regularity.  ( Source: https://www.quantamagazine.org/hyperuniformity-found-in-birds-math-and-physics-20160712/ )

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Avian Eye – the High Technology of Nature