Hi, we have approx. 14-15 large black birds (at least a 6ft wing span) perched in our backyard tree. They do appear to be all black, at first we thought they were black vultures, but through binoculars they don't appear to have a grey head like the ones we have seen in pics. They fly back and forth between our tree and our neighbors tree flapping their wings only when they take off. Help if you can...thank you!!