Re: How do birds know which direction to fly?

Nice question. But that is still a mystery to be solved, for it is linked 
to the migratory habits of birds. There have been many theories which have 
been put forward. Some say that it is the sun, while some have argued out 
that the earth's magnetic field is responsible for setting a particular 
direction in which to fly.
About turning of a flock of birds altogether, it is still as ambiguous. To 
tell you what I believe, it might be that if the leading bird takes a turn, 
the slight difference in air pressure created due to the flapping of wings 
is sensed by other birds, and they turn together. Do not take this as the 
complete answer, since this is still a hypothesis. There might be many more 
lying around!