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 spawns in the Overworld.They're also the only mob that has absolutely no impact on gameplay, since they aren't hostile, don't drop any items, and cannot be interacted with.

Appearance
Bats are visually the second smallest mob in the game, being only slightly larger than baby chicken. 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 large caverns. 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. It seems Bats don't need a solid block to spawn on, 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.

Bugs

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

Gallery
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