Hints and tips from ancient Suffolk folklore, no 432:

When I put a ball of fat out for the birds, it's usually only a day or two before the local family of magpies demolish it either by hooking it up onto a branch for the whole pack to attack at their leisure, or by ripping the bag so it falls to the ground. To counter this, I now push the top of the bag through the hole in an old CD before hanging it. This sways and tilts if a magpie tries to land on it, and so far has been very effective in keeping them away. Smaller birds, which arrive from below or on a level aren't put off at all.

I favour Donovan CDs for this, as after a while they are rendered completely unplayable.