Ferns are one of the vascular plant members that reproduce through spores, i.e., they do not produce flowers or seeds. However, they do have leaves, stems (rhizomes) and roots.
Some varieties are often used as food in some parts of the world especially the fiddleheads of bracken, ostrich, and cinnamon ferns, while others have cultural and mythical significance.
These plants contain various important nutrients. For instance, their fiddlehead greens have carbs, proteins, vitamins such as vitamin A, B2, B3, C, minerals like copper, potassium, phosphorous, manganese, zinc, iron, and so on.
Do rabbits eat ferns?
Yes. They are among the perennial plants that are moderately damaged by rabbits . Some such as the maidenhair fern are safe.
However, the Bracken, California, Cloak, Jimmy, Plumosa, Asparagus and Sprengeri are poisonous and should not be fed to these pets. They contain ptaquiloside and thiaminase which will damage your rabbits bone marrow, cause hematuria and anemia 
Since you may not know the different fern varieties to determine which ones are safe and which ones are not safe. We do not recommend giving these plants to your bunnies.
Some domestic bunnies may nibble almost anything given to them, unlike the wild ones. Therefore, if they often go to the garden, keep them off these plants.
There are many other leafy greens including basil, Bok Choy, celery, collard greens, kale, mint, mustard greens, parsley, watercress, mint and so on that can form part of the fresh foods you give to this pet.
Rabbit’s diet should consist of over 80% hay and only about 10-15% fresh foods with the rest being high fiber pellets.