I encourage the crows to hang around. I've seen them in action against a golden eagle. The crows always have a lookout in the trees and since my chickens have learned how to "speak" crow, the crows provide an early warning system for the chickens. It used to be, back in the summer and fall, that the crows would wait until the chickens were done with their scratch before they landed and tried to find leftover tidbits. Not anymore. I just looked out and saw I have three new members in my flocks. Three crows, eating right along with the chickens. Oh well, an extra bit of scratch is cheap protection for my chickens.