I don't know

I don't know why they do it ............

I don't know how they get the message out that they are about to start............

I don't know where they come from ............

I don't know how they decide to stop ............

I don't know where they go to............

I only know that it happens at a certain time of the year and I enjoy watching them do it so gracefully without bumping into each other.
:-)