The bat AI has a directional asymmetry. They are more likely to fly south or east than north or west. I haven't noticed problems with other mobs, but I haven't tested extensively.
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@unknown no need, in the description is says where they want to go:
They are more likely to fly south or east
also, could you find the ticket describing the south-east rule?
It's an article on the minecraft wiki: http://minecraft.gamepedia.com/South-east_rule
Also see MC-89509
Is this still an issue in the most recent versions (currently that is 1.10.2, or 16w43a) of Minecraft? If so, please update the affected versions and help us keeping this ticket updated from time to time. If you are the owner/reporter of this ticket, you can modify the affected version(s) yourself.
I first noticed this bug with villagers. I tested in 1.11 using villagers rather than bats. I created the enclosure in the attached picture, and spawned villagers in the middle. After about an hour, I would say they are close to being evenly distributed.
Note, there may have been (or still be) multiple bugs with the same/similar symptom; for example, back in the days, villagers (and many others) were affected by two bugs (at the same time) giving similar effect. Also, if some mobs use different class (code), fixing one leaves other version fixed while others still bugged. Considering Minecraft's bug fixing history, I'd test the bats, too, just in case they are using different piece of code than villagers.
One report too late. Oh well.