I was on the telephone with other bloggers when a lot of crows flew into the front yard and sat for a while.
They came and went three times in about half an hour. The last time, everyone left, even the stragglers. As far as I know, crows form murders if they are protecting or attacking something.
They have a real spite on their cousin, the raven and will gang up on any that flies into their territory. Otherwise, they are not an aggressive or dangerous bird. They do love dead bodies and along with vultures are the garbage collectors of the avian world. Everything and everyone needs a trash collector.