I have a gang of crows that hang out in the woods/marsh behind the coop. They are, to me, some of the best friends my birds have. I too, have seen the crows relentlessly drive hawks away from the area. My birds seem to have identified the crow's hawk warning call. The crows, when around call to each other through the day. But there is a certain call, the hawk call, that when the chickens hear it, their heads pop up, and they haul it to cover. No hesitation, they GO. They learned it.
I have done nothing to draw the crows here. They were here before me. They have never bothered the chickens. Although, they can be a nuisance, when I plant the garden,, they really like fresh planted corn.