If you aren't certain if the bunnies will like the fiber, just put it in their cage and let them decide if they want to put it in their nest box. Does seem to like putting stuff into their box, it keeps them busy and gives them something to do so they won't have time to get fussilated about the upcoming litter. Also, if it is a fiber or material I'm not sure about, I just put it where the dog can reach it if she wants it. The seem to like all sorts of fresh grasses, dried grasses, leaves, bunny hair and sometimes sheep's wool, but the dried grasses and bunny fiber seem the preferred options.