When people talk about eagle vision, it is a perfect metaphor for the ultimate visual system. Eagles, hawks, and birds of prey have the best sight of any animal. Just the need to fly requires excellent vision, but when you have to spot your dinner on the ground and dive after it at incredible speeds, you will go hungry if you don’t see exceptionally well.
First of all, their eyes are enormous. Our eyes take up 2 percent of our face, but these birds can have eyes that are 15-20% of their head. Sometimes, their eyes weigh more than their brain! They also have an extra eyelid that is transparent. They can blink and still keep seeing. Inside their retinas, there are other differences from us. We have two special receptor cells in our retinas to convert light to electrical signals—the rods and cones. Birds have an extra receptor cell that probably allows them to see colors that we could not even describe.
It doesn’t stop there. As Sy Montgomery notes in her 2010 book, Birdology, the bird’s retinas are densely packed with these receptor cells. In our eyes, a special area of the retina called the fovea contains our best sharpest vision. We tend to have 200,000 cones/mm2 but eagles could have over a 1,000,000 cones/mm2. This would create a world of such detail that we would need microscopes to approximate what they see. Remember when you saw your first HD television next to a regular picture. Imagine a few levels of detail beyond that and you can start to think how these birds see the world.