Sight is the dominant sense in birds, as it is with people, because they are very mobile animals within a three-dimensional world and need precise information about their surroundings as they fly and hop from place to place.
There are many similarities in the structure and capability of the human and avian eyes, but there are also some differences which give the birds some very significant improvements.
The bird's eye comprises two main sections or chambers divided by the sclerotic plate, which is a ring of small bones, and lens. A clear liquid fills each chamber. The muscles anchor at one end to the sclerotic plate, at the other end to the lens and cornea. By contracting or relaxing these muscles, the shape of the lens and cornea change, and the bird can focus on an object.
The iris is a thin sheet of muscle in front of the lens that can control the amount of light entering the eye. The iris contains colour pigments that give the eye its colour, the common colours being brown, red, white yellow, green, and pale blue. The opening in the iris is the pupil.
The sclera is a tough, white layer that forms the "white" of the eye and gives the eye its general shape, and the choroid contains a rich blood supply that carries nutrients around the eye. The retina is the inner surface of the eye, and this contains densely packed, light-sensitive nerve cells called "rods" and "cones". The fovea is the point where the cells are at their densest and the image is at its sharpest.
Cones are specialised for seeing colour in daytime and rods for seeing in dim light or at night time. Consequently, rods are more predominant in nocturnal species, and cones in diurnal (daytime) species. In some species, such as Kestrels, and thrushes and warblers, some of the cones are sensitive to ultraviolet light. This is useful for the Kestrels in locating voles, their favourite food, which leave a trail of urine wherever they go, and the urine shows up in ultra-violet light and for thrushes and warblers to locate UV reflecting berries in the autumn.
The pectin is unique among birds and reptiles and ornithologists do not yet fully understand its purpose. This structure protrudes from the retina in to the rear chamber and contains a rich blood supply. One possibility is that it supplies nutrients and oxygen to the retina, thus aiding the choroid in its function, because the avian retina, unlike human retinas, does not contain blood vessels.
Most birds have binocular vision, like ours, so that they can precisely judge distances. Raptors, such as hawks and owls, have especially well developed binocular vision and their eyes are towards the front of the head. Indeed, owls' eyes are forward facing. Small birds usually have their eyes more to the side of the head so that they have better all-round vision and can see danger in all directions.
Birds such as pigeons and waterfowl, whose eyes are side facing, have so little binocular vision that rely on apparent motion between close and distant objects to judge distance. That is, imagine that you are travelling along in a car or train, the scenery closest to you seems to pass in a blur while the scenery on the horizon appears almost motionless. This may explain while waterfowl constantly bob their heads when standing still or walking, while pigeons move theirs back and forth.
Visual acuity is the ability to discern fine detail and this is a relative feature: Dunnocks, for example, must be able to distinguish fine seeds on the ground, while a Sparrowhawk must be able to see small prey from far above the ground. To achieve this, birds have large eyes, larger in relation to their heads than ours; for example, the eye of a Starling is fifteen times larger than ours is relative to the head. As a result, their visual acuity is typically at least two or three times better than ours or, in the case of an eagle, as much as 5 times better. SO, if you can read a car license plate at 20 metres (22 yards), an eagle could read it 100 metres away!
The Starling is of further interest because when it is probing the ground with its bill, it can rotate its eyes forward and look along the length of the bill to look for prey, increase its binocular vision and judge distance precisely.
An owl's eyeball is more like a tube and this increases the focal length of the eye, and gives it telescopic vision, and large irises that can open its pupils very wide and gather lots of light. Both these adaptations help it to see small prey from far away and in dim light. In total darkness, owls, like the Barn Owl and Tawny Owl, rely on their fantastic hearing to pin-point prey.
There is yet another fascinating adaptation in birds, though it has not yet been fully proven, and that is that some of the nerve cells in the eye are sensitive to magnetism. If this is the case, then they eye may also be the birds compass.