Bat

Bats are passive mobs that spawn naturally in caverns or other large enclosed spaces. Bats are the only passive mob able to fly, and the only flying mob that naturally spawns in the Overworld. They will often emit a squeaking noise, but do not do many other things that can affect gameplay.

Appearance
Bats are visually the third smallest mob in the game, being only slightly larger than baby chickens and Silverfish. They are hard to see in the unlit environment in which they reside. They have dark brown skin, small wings and little feet.

Spawning
Bats will spawn in darkness. They can also be spawned with their spawn egg. Bats may spawn in an unlit house if its interior is large enough.

On superflat worlds, Bats have been observed to spawn in even 1x1 spaces that are unlit, such as in mob traps, possibly because the code that spawns them cannot find any other place to spawn them. This indicates that Bats do not require a large cave to spawn, but large areas do have a much higher chance of causing them to spawn. Bats will not spawn on glass or slabs, but could spawn randomly in your world if the light level is low enough and the Bat doesn't have enough spawning space.

Behavior
Bats sleep during the day and become active at night. A bat will hang upside down when it is idle, but once the player approaches, the bat will fly away. Bats cannot hang on transparent or non-solid blocks. When placed in a minecart, bats can move the minecart around on their own.

A lot of times, Bats will squeak (similar to its real life function of echolocation). By listening for bat squeaks, the player might sometimes be able to find caverns, which can be useful for obtaining resources.

Although the bat is a passive mob, they still despawn when going too far away, like Zombies and Zombie Pigmen.

Bats do not seem to care about lava, and if there are lava streams coming from the ceiling, they may fly into one and burn up. Most bats, however, are safe from lava, both because lava streams are uncommon and because lava gives off light, and bats spawn in the dark.

Bugs

 * There are odd collision bugs with bats and blocks; specifically, they might be able to move up through diagonal roof of glass.
 * Sometimes bats hang in mid-air.

Trivia

 * Despite the conditions being hard to find normally, it is possible for bats to drown if trapped in water.
 * Bats seem to be able to spawn in mushroom biomes
 * Since bats use echolocation to "see", they will notice players even when they are invisible.
 * Bats tend to fly to the east whenever given a choice.
 * Bats were originally planned to be neutral. It is unknown if this will be implemented or not.
 * Bats are one of the few mobs that fly, the others being Ghasts, Blazes, the Wither, and the Ender Dragon.

Gallery
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