I recommend you watch the area around the den, you will find bones, fur and feathers etc and be able to tell what the adult fox is killing and bringing back to the den to feed the young once they are weaned and to feed the Vixen when she is nursing because she needs a lot of food to produce enough milk for the pups, it can be pretty interesting. It is also fun to watch the pups when they start to venture out of the den. Unless you have free range chickens or ducks, I wouldn't worry much about having a den on your property