Heavy Bird Flock in Trees Calling All Lost Birds

When a flock of birds lands in a tree, and all of them start chirping simultaneously, why do they do that? Can they make out each others individual voice? Or is it just a "I'm sitting here" kind of message?

The birds are most likely starlings or
a similar flocking bird gathering in a
sheltered location such as a tree. They
are calling to gather any other lost birds
of the same species who are looking for
a flock and a safe place to shelter, as
well as to establish dominance within
the flock.