This is a lesson I just keep learning over and over again. We have lost chickens to various kinds of predators several times. The last time I made a stockade for the chickens: fence wire top, sides, gate, along the ground, etc. But it's kinda small (10x10) and the hens really love to get out and scratch around the haystack and goat pen. But if I don't keep them in the stockade, eventually something gets them. The owl seems to be gone now, and from what Ang said, it sounds like he was the culprit. But the remaining 2 hens are going to stay locked up. I may cover the top of the goat pen with fence wire also, both to protect the chickens and to keep big predators like mountain lions from getting my goats.
The last time this happened I figured it was coyotes. So I made the fence much higher (about 7 ft). I also made a big live trap out of pallets and baited it with a with meaty bone. It had a door that would slide shut when anything got inside and tugged on the bone. I caught my dog twice, and various neigborhood dogs, but never a coyote. I think I caught a raccon once, but he got thru a wide spot in the trap and escaped. He left some fur behind. He never went in the trap again. I finally decommisioned the trap. If the problem had been neighborhood dogs, tho, the trap would have worked like a charm. They never wised up to it.
We live in the midst of a big forest. Other than the goat/chicken pen and a yard fence around the house, I don't have a fence around my place. I don't want to kill critters just passing thru, unless they are in my yard or pen, or trying to get in.