Could be Racoons, but here were we are from in ohio we have alot of mink problems(captive bred farms also). We have lost every flock we have ever had to mink, so bad we wont raise any more. Mink would be my guess as to you problem, those little ******* will kill eat head, drink the blood and leave the body lay. All the hundreds of chickens we have lost to them over the years, have not found an ounce of blood. They topically wait till dark and will slaughter a whole flock in a matter of minutes. Like fox they are extremely hard to trap unless you have a very knowledgeable trapper in your area. Like other have said dogs seam to be the best way to keep them at bay. We had the best luck raising birds when the were beside our beagle kennel. Free range birds dont stand a chance and once they know they can get food there they will continue to return. Good luck!!!