There are almost a million rods per square millimeter in some owls while we have only around 200,000 per square millimeter. That certainly explains why owls can see so much better at night than we can. Another reason nocturnal birds see better at night than humans is that these birds' pupils are larger. These huge pupils allow more light to come into the eyes than our smaller pupils do. Cells called cones are also present in eyes.
Cones allow the brain to perceive colors.
Nocturnal birds who have the extremely high concentration of rods will have very few cones - it is much more important to be able to see in low light and not very important to distinguish colors. On the other hand, birds who forage and feed in the daylight see colors very clearly from far off. It's been said that a hummingbird can spot red flowers from over a half mile away. There are also foveae , which are zones of maximum visual sharpness. People have one fovea.