sorry to hear about losing so many birds. it is always a constant battle here in michigan, if you do have mink there i would lean towards that being the the predator killing and leaving the body's though i have found they eat the heads. the best way i have found of catching them is live trap with one of the body's, they come back for them. it could also be a bird of prey but they seem to take the body's. i have surprisingly caught them in live traps too. i would strongly suggest the investment in some sorts of traps. i have no idea of the laws over there but live traps are the only sure way not to kill some thing unintentionally. expectantly if you have your dog out there. best of luck.