How do birds fly in unison?

Question

How do the birds know when to turn and in what direction as a flock? What is the mechanism by which they do this, for example: by sight, sound, etc.?

Answer 1

Great explanation here: http://www.straightdope.com/mailbag/mflockswoop.html

Answer 2

It has been demonstrated with computer programs and followup research that "flocking" can be done with very simple mechanisms involving about 4 rules like "Keep the distance from birds around you the same", "Keep birds all around (move from the outside to the middle" "save energy by positioning yourself behind the bird in front of you."
Sight
The same process works with schools of fish.
As a result, what was assumed to be a very complicated process of communication or detection is much simplified and program a swarm, flock, or school into a computer realistically is much easier.