Sometimes, they spawn on trees and will follow the player, even to the point where they are "planted" on air. From what I heard, they're supposed to spawn on the ceiling, or at least in places where they can effectively attack the player.

What they were intended to do is to spawn as a normal mob, and occasionally scream and damage nearby players.

Well, they hang on the ceiling, but occasionally they are found hanging off of the bottom of trees and move off the trees just to get to the player.