It depends on the fern. Do you have any idea what the particular ferns are? Some non-toxic ferns that I've read are: boston, asparagus, bird's nest, maidenhair, cinnamon.
All true ferns should be considered dangerous; most contain a group of compounds called glycosides that are toxic, especially when eaten regularly. These compounds are the reason that when cooking fiddleheads one changes the boiling water twice to leach out the toxins.
Asparagus "ferns", are not true ferns, but a relative of the edible asparagus, however, they have high levels of another slow poison and should not be a regular part of the diet of any animal.
Okay...
I know ferns can be considered poisonous and everything, but I think they are a great plant to have in the enclosure (depending on the variety) and I've seen many pictures and posted about ferns being kept in the enclosure... and it doesn't seem to be the type of plant torts WANT to eat... Just me