All birds are endowed with exceptional visual capacities. These are the animals most dependent of all on the sense of sight. Living up in the sky, they have to spot their food, their territory and their enemies as rapidly as possible. In relation to their overall size their eyes are very big. They differ greatly from our own. We only see clearly objects directly in the axis of our pupils, and we have to turn our head or our eyes to take in a wider area. Birds have a far broader field of vision.
In birds of prey, like the eagle, the eye is very deep and almost tubular. Because of this the image formed on the retina (the sensitive part of the eye) is much larger. This enables the bird to pick out much more detail. In addition, for every square millimeter on the eagle's retina, there are three times as many sensitive organs as there are on a man's.
Their colour vision is highly developed and heightens their perception of detail.
It is said that an eagle 300 feet up can distinguish between two dots half an inch apart!
In birds each eye has independent vision. The vision of the two eyes is not fused into a single image as ours is. They ought, because of this, to be bad judges of distance. Since- quite the opposite is true, they must have other means of seeing in relief: this can perhaps be accounted for by the comb-shaped crest which projects out above their retinas and is highly developed in the eyes of birds of prey.