Bat

Bats are passive mobs that were added in Update 0.11.0. They spawn naturally underground. They do not drop anything.

SPAWN
They spawn anywhere at a light level of 4 or less on opaque blocks below layer 63.

BEHAVIOR
During the day, bats will hang onto blocks and 'sleep'. They will however, start flying if the player approaches them. Bats cannot hang onto transparent or non-solid blocks, such as Glass.

Trivia

 * Bats make squeaking noises, making it easier to locate caves, strongholds, dungeons, and other underground structures.
 * Although they are a passive mob, they despawn after the player moves too far away like hostile mobs and like Ocelots.
 * This is because they are coded in the game as hostile mobs.
 * Bats don't recognize lava as an obstruction, so they can easily fly into it and catch fire and die.
 * They are extremely hard to kill (or it takes time to kill them), because they can fly in any direction, up and down, and they are really fast.
 * Bats are the only passive mob in the game currently that doesn't drop anything.
 * A Bat can be caught with a Fishing Rod.
 * They are one of the two flying mobs in MCPE.