I didn't use CD's, I used the lids off of cans. I have one of those openers that leaves no sharp edges. Collected lids from cans (and the lids from those frosting cups in whompum cinnamon rolls) all last winter. Then tied them around my sunflowers and garden beds after dh punched nail holes in them. They work because they reflect the sun, plus make bangly noises when they bump each other.
I've seen hawks up high, but not close to our yard. DH thinks it's the rubber owl he put on the swing set that keeps them away, though. But I know my lids kept little birds off my garden and sunflowers, and kept the squirrels away, too. No easy food had to have kept the hawk from being interested as well.
Since the little birds pooped all over DH's owl, I don't think the hawk would take it seriously, either...