A formation of objects swirling and changing brightness.

I first noticed the formation when I was at a red light heading north bound on West Bernardo Dr about to turn left (or West) on Rancho Bernardo Road.  Their were about 10 or 15 objects in the sky in a formation.  
The objects seemed to be rotating in spiral fashion, and at first I thought they were a group of sky divers, but then I realized that they weren’t actually falling.  As they swirled around they would change from bright white to almost invisible to a dark color then back to invisible then back to white.  This cycle made me think that maybe it was a group of birds that were white on the top and dark underneath.  
I thought maybe as they were flying in circles the whites of their back were being reflected by the sun then the dark of the birds bellies were making them look black.  Some of the objects would  come close together and some would separate from the group as a whole.  After about 15 min they slowly dispersed to the east and gradually disappeared.