[EBB Sightings] Mystery birds

I saw an extraordinary sight on Monday, around 1:00 pm: about 40 large 
pale birds with very dark primary and secondary feathers flying 
together, continuously swirling in a funnel shape as though they were 
kettling but not gaining altitude and moving pretty rapidly from north 
to south above Berkeley.  They didn't have the jizz of a raptor, though 
I wouldn't rule that out entirely.  Unfortunately, I was in a meeting, 
looking out the window, and so I couldn't leap up for a better view.

I'm hoping someone can identify what I saw, based on the behavior and 
distinctive markings -

Thanks for your help -