We live in the middle of 10's of thousands of acres of National Forest and Wilderness lands in Northern California. Bear, Mountain Lion, Coyote, Fox, Racoon, Skunk, Ring Tailed Cat, Fisher and many other of weasal family as well as hawks, falcons, osprey and eagles. (Actually found the eagles the most confounding). When you have such a good food source like chickens you will have some hungry critter do it's best to eat them. The best word of advise is a serious fence. We have about 2 acres fenced with 2X4 X 5 ft. field fencing as well as a smaller more controllable cross fenced area near the Chicken house with its closed in run. I make regular inspections of where wire meets the ground to see if any attempts at digging under it have occured We have had some minor intusions in the past but each time I was able to make changes to insure the saftey of my flock. Vigilance is the minimum expected of us as protector of our flocks.On occasion I have had to dispatch or trap persistent predators. Having chickens free range makes for happy and healthier chickens but it also exposes them to more dangers. Sorry for your loss. Don't give up