Birds, eg starlings, can flock in the tens of thousands - how come they don't all crash into each other?

Question

http://news.bbc.co.uk/1/hi/scotland/south_of_scotland/7252568.stm

Answer 1

They move as with one mind.  I have never heard a reason but have heard it called a mystery many times.  Fish use lateral lines to feel pressure waves.  It would seem reasonable that birds use visual cues and must all focus on some leader or focus on the group as a whole.  It is a great unanswered, as far as I know, question to contemplate.

Answer 2

Ha, that's nothing!  Bats flock in the "millions" as they leave their caves and swirl about in a vortex for several minutes before exiting into the night.  One cave in Texas contains the largest population of mammals anywhere in the world; 40,000,000 Mexican Free Tailed Bats in the summer.  Fish can also swim in "schools" so large they are almost uncountable.

Occasionally these animals DO hit one another slightly when in such masses.  Scientist think that possibly these animals "feel" the pressure of the ones next to them and turn or dive when the others do.  To my knowledge scientists don't really know this for sure.  It is an animal mass behavior that is still being studied.