Aww man I'm sorry, what unlucky timing with the bat raid. I've played for 1000+ hours and never even seen the bat event. RNG at its finest huh?

Unfortauntely, once you kill your first bat, and bonemass, the bat event becomes possible which changes how you must protect your animals. Your boar and chickens will be vulnerable from now on unless they are inside of a building. So, to protect them, they must be totally enclosed. No windows. No open sky. Nothing.

Or never kill a bat. If you never kill a bat in the caves; you'll never get the bat event. They aren't necessary to fight so its entirely possible to avoid them