Do you leave your coop open after dark? Your chickens need to be locked up at dusk inside their coop. The fox will return and kill more of your flock if there is any way it can get inside your coop. If you had them locked up, the fox could have burrowed in through a hole. 1/2 hardware cloth fencings should be used on windows and some make a 1 foot wide apron along the outside of the coop to prevent burrowing. Sorry for your loss.