They can easily jump the fences.
@LilSnakeHips and @MayaMoon1397 in addition to the remarks @jpkarlsen made. You can upgrade your hen houses/ chicken coops and when your sim succeeded the nerd brain aspiration and gained the trait that comes with it, it should be fairly easy. With this upgrades your hen houses will be equipped with a nasty fox alarm. That might come in handy.
Have fun playing. 😉
I have one of the big stone fences around my coop with a solid locked gate and have yet to see a fox in there. They circle but don't go in.
I don't know if that means they CAN'T, just that I've been playing for some time and they don't.
I did, however, have to lock my front door to keep them out of my house. :P
