My neighbour years and years ago told me she couldn't figure out what was killing some of her hens - they were headless, and then ragged, killed through the day time inside a complete wire pen... Eventually watched a fox sit by the pen and a hen put its head through the wire = fox bit it off! The realised the rest of damage was done by other hens pecking at the body. (Thinking outside the 'box' (pen?))
Cats sometimes like chewing skulls of birds I've found. Small mustalids like killing by back of neck, but would be unlikely to put a hole in their skull preferentually over other softer areas.
I'd set traps if I were you, some live capture, maybe fen traps (in tunnels for small mustalids/rats), increase electric wire componant, consider a camera so that you can find out what you're up against? How about sprinkling a boundary of soft sand around the outside of the pen so you can read off some paw prints and find out the entry points? I keep traps set all the time around my hens and have caught a few interested parties (rats, ferrets, ferrel cats) before they've had a chance to do any damage... touch wood!