NOTE: Starly has the Keen Eye Ability regardless of where it is caught in Generation V.
A Pokémon with Keen Eye cannot have its Accuracy Statistic lowered, causing the moves Flash, Kinesis, Sand-Attack and Smokescreen to fail. Moves that reduce Accuracy as a side-effect, such as Flash Cannon or Mud-Slap, will still do damage but the side-effect of Accuracy reduction won't take place.
If a Pokémon with Keen Eye is in the party's lead slot, the chances of encountering a lower level Wild Pokémon fall by 50%. This only applies to Pokémon that are up to five levels less than the level of the Pokémon that has Keen Eye.