We had a group of a dozen or so cinnamon ferns growing for over a decade in our yard. About a month ago, we discovered that at least half of them had either been trampled or eaten nearly to the ground. We've never had problems with animals in the yard before (our pets don't bother them, and the yard is fenced), so we're at a loss as to the cause. The larger question is whether they will regrow. It's an early spring, and I'm already seeing the first tiny new growth on some of my ferns that die back over the winter. I'm keeping an eye on the damaged ferns but haven't seen any clear signs of growth. I will replant them if necessary but don't want to rush into that if they'll come back.
Damaged Cinnamon Ferns
Certified GKH Gardening Expert
It is difficult to say what caused the damage to your ferns. I do know plenty of gardeners that set up wildlife cameras in their gardens to 'discover who did it'.
Look for signs of tracks in the soil. that may be your biggest clue.
I have heard of birds doing significant damage to tender perennials, including ferns.
If birds are the issue you could try netting covering the plants until they are larger.