All along I felt it had something to do with eq/volcanic activity but didn't know about the quartz.

Crap, you're brilliant! Your theory makes the most sense. Thank-you.

Also why does it make a difference that the birds were mainly the same type? Birds fly in flocks and it just so happens that flock was in the wrong place at the wrong time. I really think it would have happened to any type of flock if they had been at that same spot.