cormorants

The cormorant army's convening
It's stretching across the bay.
It looks like they're planning a fly-by
But they have to decide which way.

Now a signal  comes through and they line up
One call and they all sally forth, -
Heading like hell for the southland,
then they all spin around and head north.

I am awed by their disciplined order,
as their passages darken the sky.
But I can't help asking the question:
Who gives the signal - and why?