That is a lot of them, and you say you have never seen them in your yard, I wonder if that is bad news as their in a possible change in their food supply.
Drought, and I think their favorite old tree was cut down. I did some reading on them, interesting what they due.
Is there a carcass around? I would not feed them either besides what you usually feed will not be on their wish list.
Now you know how Edgar Allen Poe felt, "quoth the raven, nevermore." This picture is pretty awesome, since it isn't in my yard.