Good afternoon, It is possible that the foxes may be entering your yard to go after something attractive to them (i.e. the scent of food or available prey). Make sure that any dog food and water is brought in at night. If pet food is stored outside, make sure that it is stored in wildlife proof containers. Securely store garbage. If you feed birds or squirrels, the seed might attract other animals (mice, voles, etc.) which in turn could be attracting the foxes.
Paul Brewer, Illinois Department of Natural Resources District Wildlife Biologist, also recommends inspecting the fence to make sure there are no holes or spots under the fence where foxes can gain entry. It is important to make sure the fence has small enough mesh size to exclude foxes. Depending on the type of fence that you have, it might be possible to add inexpensive chicken wire or other very small mesh fence around the bottom 3' or so to make the yard less accessible to the foxes. The cost of that would depend on the size of the yard of course. If the food/attraction is great enough the foxes might dig under the fence as well.
If removing whatever is attracting the foxes does not work and the foxes continue to be a problem, you could hire a nuisance wildlife control operator to trap the foxes. Contact information for operators in your area are available on the 'Find a Professional' page of this website. If you have more questions please feel free to e-mail me.
