What is vision mechanism in birds?

Actually, we know a lot more know about bird vision than we used to. For one thing, we know that most birds see color far better than humans do, since the human eye has three cones (or color receptors) but birds have four. For another thing, many species-- especially Birds of Prey-- have a far greater ability to see even small objects at a distance; some can even see an object as far as a mile away. If your library or bookstore has the book "Bird Sense: What It's Like to Be a Bird" by Tim Birkhead, it's well worth reading, as he shares some of the latest research on the hearing, vision, and other senses of all species of birds. I also enclose a link specifically about the vision of birds of prey.