From pbs.org: Eagles can spot an animal the size of a rabbit from up to two miles away. Their eyes, which are very large in proportion to their heads, are densely packed with sensory cells. While humans typically have 200,000 light-sensitive cells per square millimeter of retina, eagles may have 1 million — five times more. Similarly, while humans have only one fovea, a funnel-shaped part of the retina where vision is sharpest, eagles have two. Finally, where people see just three basic colors, eagles see five, enabling them to pick out even well-camouflaged prey.