Look carefully, and you'll see other large birds sharing their airspace. (e.g.: Bald Eagles, Ospreys, Redtail Hawks).
A friend who soars gliders in those updrafts says the birds "chat" among themselves.
I love watching raptors soar. You see them frequently flapping their wings to find new thermals. You hardly ever see a vulture flap its wings. Next time you see some, watch for that. They generally only flap wings to get airborne and land. They are that good at soaring.