Deer are colored blind, seeing the world in monochromatic tones and shades of gray. Even the blink of a human eyelid can be enough to send the alerted deer running off to safety. The sense of smell plays a very important role in a deer's world. Observations show that a distance of about one-third of a mile is the maximum range scent can be detected by deer. Also, their hearing is excellent.
Ears are constantly in motion, picking up sounds of possible danger.