Bat

Bats are passive mobs that were added in Update 0.11.0. They spawn naturally underground. They do not drop anything when killed and they do not harm the player.

Spawn
Bats 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

 * Although they are passive mobs, they despawn after the player moves too far away from them.
 * This is because they are coded into the game as hostile mobs.
 * Bats don't recognize lava as an obstruction, so they can easily fly into it, catch fire and die.
 * They can be very hard to kill as their hit-box is smaller than a block and they can fly in all directions.
 * They are one of the tree flying mobs in Minecraft PE, the others being the blaze and the Ghast.
 * They are the second weakest and smallest mob in the game, after the Chicken.
 * The bat is the only passive mob in the game to not drop a single item.