The ferrets have eaten a good number of magpies over the years without any ill effects, but I don't normally let them keep their catch. I remove any bird as soon as I discover it since they are loaded with parasites and possible diseases. Sometimes the feast is already over by the time I find out, and all that's left is basically just the feathers. Most everything else is devoured. In the pic below, from another magpie kill, the beak and part of the cranium were the only remains.