A person with eagle eyes notices everything and every detail, but having eagle eyes is as much about sight as it is insight. Like most idioms, the direct meaning of this saying is different from the implied meaning. Idioms use a comparison or metaphor for the sake of clarifying an idea. While they are usually clichés, idioms can still be used as effective forms of communication.
Idioms are sayings that mean something other than the literal meaning of the specific words they entail. They are frequently metaphors or similes, and due to their common use the meanings are implicitly understood by most people. The term "eagle eyes" is a metaphor for someone with acute powers of observation and insight. This person has a remarkably keen ability to direct his or her focus on an object. The idiom "eagle eyes" also suggests a predatory sense of perceptiveness due to the metaphorical allusion to a bird of prey.
The reference to an eagle is an apt one when describing someone with sharp powers of observation. Eagles are known for their remarkable vision. When hunting, these birds of prey frequently find the highest vantage point, like a tall leafless tree for instance, so that their vision is unhindered. Some eagles can spot a fish in water from several hundred feet in the air, even while flying. Since the color on the top of a fish usually blends with the color of the water this is no small accomplishment.
Idioms using animals are very common. Some examples are “bigger fish to fry,” which means that a person has more important matters to deal with than what is at hand. Another example is “beating a dead horse,” which means that trying to change something or someone’s opinion is to no avail, because it cannot be done. A person who is “all bark and no bite” does not back up their angry words with action.
Most idioms are timeworn sayings that people have heard over and over again. They do not necessarily make for great writing if someone is trying to impress other people with his or her powers of metaphorical creativity. Idioms can certainly get the point across though, and can provide an economical use of words. It is much more fun and colorful to say someone looks like “a deer caught in the headlights” than to tell someone that he or she appears to be caught off guard and cannot figure out how to react or what to say.