I live in a suburban area. A few weeks ago we heard a large murder of crows, they were harrassing a red-tailed hawk. After some time, they seemed to tire and most of them left but the hawk continued to fly from one tree to the other. We have also had several visits from a Cooper's Hawk. The latest, a couple weeks ago. I had my eight hens out in the yard for some free range time. Six of them were pecking and scratching in my garden, about 30' from the coop, and two of them were hanging around the coop. Suddenly I noticed that none of them were moving at all and I could hear a strange sound a couple of them were making. It was a low, whistling/purring sound. There was a Cooper's Hawk about 20' up in a tree right above their coop. Once I saw him I scared him off. It is rather amazing when they do that. I have seen it three or four times when a raptor is around. One time there was some shrubbery that was fairly close to them so they ran under it, but every other time they have seen a hawk they just froze and a couple of them made that whistling/purring sound. I assume it must be some type of alarm or warning vocalization. The only time I have ever heard them make that sound was when a hawk was nearby. I have been very fortunate that the hawks have never made an attempt. Nevertheless, we never let the girls out without supervision.