I'm not sure if CDs will scare off an owl or not. The best thing to do is put up your chickens a little earlier so the owl doesn't get a chance to grab one. Do you have them in a run?If you can cover the run that would be best.
If it is a Great Horned Owl, then it is a danger to chickens-- all sizes. If it is a Barred Owl or of course, Screech Owl, then NOT a danger. I don't know about Barn Owls-- anybody else? All are easy to tell apart by sight from a distance or by their sounds.
We have a family of barred owls. The parents nest in our huge oak tree trunk every spring and then we get to watch their babies grow up. When my chickens see a large shadow, they startle, but then when they realize it is the owls, they just go back to scratching. The owls don't bother them.
We have never had a problem with mice or rats with these big guys!