Raptors are believed to have the keenest eyesight in nature because of the size of the eyeball and the eye muscles designed for rapid focus. Diurnal raptors have full color vision and two concentrations of cones (one directed to the side and one directed forward) in each eye, which control color perception.
The sharpest point of vision at these concentrations is called the fovea. When the fovea works in unison, they give raptors accurate depth perception, which is very important for birds of prey that must focus quickly when chasing moving objects.
The keenness of vision is related to the agility, size, and color of prey a raptor hunts. When a bird bobs its head, it is using its eyes like a range finder and focusing in on a specific area.
This special transparent eyelid closes laterally across the eye and is used to
• keep the eyes moist
• protect the eyes during flight
• protect the eyes when feeding themselves or their young
When humans close their eyes to blink and sleep the upper eyelid closes. Depending on the species, raptors have eyelids that close from either the bottom or top, for blinking and sleeping.
An additional form of eye protection in most raptors is a bony shield that projects above the eye. The bony shield adds protection when raptors pursue prey into brush, protects the eyes from injury while hunting, and also gives raptors a menacing appearance.