There are many products you can buy that will play predator calls, distress calls, and all manner of other things, but I'm too cheap for that stuff. The first thing I did was visit the Audubon Society and other birding websites to find recordings of hawk screams and distress calls. I found some recordings that I thought would work, and mixed them together into a five-minute long mp3, which contained the predator-alert call of the American robin, followed by the scream of the red tailed hawk, and finally by a few minutes of silence. By loading the track on an old iPhone I had lying around and setting it to repeat, the calls would play every five minutes. I'm not sure if this is too much or too little, I'll just have to watch carefully.
I got a couple of cheap computer speakers, and spliced in some stereo wire so that they had a very long lead. I then placed them in a plastic 5-gallon bucket with a hole cut out of the side (weather protection), and placed them up on top of the 1,000-gallon rain tank overlooking the blueberries. Once everything was wired together, I hit play and crossed my fingers. Success! The screech of the hawk shattered the silence. If I was a berry-eating bird and I heard that call coming from up above, I'd probably lose a feather or two. I let the track loop for almost six hours today, and I'll probably let it run for eight+ hours each day until blueberry season is over. It's not so loud that I think it will bother any of my neighbors, and I have a feeling that if it did bother anybody, some fresh-picked blueberries would probably smooth things over.
Robin Predator Warning Call
Red Tailed Hawk