Did you notice that their tails had white tips on them as they are fanned out as they fly?
I have been catching glimpses of birds like these for several years and have yet to identify them.
They are very similar in appearance to Blue Jays and to Downy and Hairy woodpeckers, but I have ruled all of those out..
From what I have seen they have a very loud booming call which sounds like a "Woo Dee Dee, Woo Dee Dee". The last two notes in the series are one semitone higher than the first.
Their call is high pitched with a whistle timbre and they are very graceful. I always see them gliding slowly and usually in packs of 3-5.