It's a mink. I had EXACTLY the same thing happen to my beloved birds, 7 of my 8 coturnix were there one night, gone the next. Well, I say gone, but the bodies were still there, it was just the heads that were missing. It got in through a mouse hole which I didn't fill in because it's owner was doing quite a nice job of cleaning up the dropped food in the run. There was one bird left, (who is now being kept in the house) and she was only still alive because she managed to keep out of sight on top of the coop. Anyway, we called in the pest man, and eventually after catching numerous rats with dead supermarket birds in traps (which had their heads ripped of by something too) the mink was brought to justice by Dolly the super-hen (known for her sparrowhawk tormenting), who one day trotted up to me with it in her mouth. After Dolly killed the mink, the dead birds stopped losing their heads, and we have not had another bird killed since.
