Did see that many on the ground before when there was a dead deer but never up in the air. They vanished before I could locate the area. I knew the general area but our streets are weird shaped.Happens all the time, but usually only for two reasons: there's a good thermal in that area, or a fresh carcass (N. American vultures don't eat rotten meat), check to see if there's a corpse, it won't be very smelly if the turkey and black vultures are interested.