i'm sorry you have a fox problem but taking pot shots at the fox won't help, even if you did kill it another will relace it very quickly, i have the same problem, i live on a farm and penning all my birds isn't an option, a good strong fence won't be any good as the fox will merely go over it, i think your only option would be to invest in electric poultry fencing, might be costly initially but the replacement of future birds will probably cost more, be aware that now the foxes know the hens are there, they will be back.