Yes, hawks are threatening, and crows probably are nesting near the field where I watched this encounter Saturday, but I think crows need more to do. They harassed this Red-tailed Hawk for several minutes, three of them, following it for maybe a quarter mile before giving up. Later, nearby in a woods, I watched crows, 10 or a dozen of them, drive a Great Horned Owl from the woods, and five minutes later come back to find another, driving it away as well. A pair of owls had been tending a nest in those woods. I was there to check on that. The nest lacked an adult bird. I assume the pair that drew the crows' attention had given up on the nesting effort. I hope they remain loyal to the woods.
From the ground it can look like hawks ignores their harassers, usually continuing to circle in its quiet glide instead of fleeing as owls will do. You can see from these photos that the hawk was very aware. In the third photo the hawk is watching another of its attackers.