Huh. I would've expected one of the 18 month old girls to step up to the role of sentinel. Do they have places they could hide under, when they free range?
I'm so sorry you lost your beautiful girl to a predator. Losing a chicken is hard, sending hugs your way :hugs :hugs :hugs . I have not ever tried a fake owl or anything of that sort, but I have not read many positive things about them. They might work for a little bit, eventually the wild...