Bats, Bats all over me...

When bats come out of a cave in large groups like
these Mexican Freetails do, they swirl out like a corkscrew.
From distance it looks like wind blowing black smoke.
But when you get closer the corkscrewing and all the little
fluttering which amounts to a really interesting sound.

Anyways, why do bats in large groups corkscrew like this
instead of flying like birds in large flocks or like fish (if you
have ever been snorkeling and been in a large school ya know
what Ima talkin about) in a school?

very diff pattern of avoiding
running into each other...

I wanna know so bat-people give it up.