I have a spayed female rabbit that was accustomed to living on her own but was in a shelter for 6 months. When I got her, I put her in a three-room cage (big room, hide, litter and eating) previously used by another rabbit; she immediately started digging in the litter area, finding things to eat. At the time, the litter area was straw over extruded sawdust.
Due to the eating, I replaced the straw with horse hay that other rabbits had already eaten all they wanted of (and moved the bowls for food and water to the big room), but the digging and eating choice bits continued. When I replace the litter, the rabbit is initially upset over the loss, but is soon digging and eating choice bits again. Recently, after a especially vigorous two day campaign during which I straightened out the litter area as much as I could several times (as well as changing it out), she seemed reclusive and not feeling well. I replaced the litter again and she returned to normal in less than a day without continuing the digging.
So the question is what does this rabbit find of value in used litter (straw or hay) and how would I keep her from overdoing it?
---------------- added in response to Allerleirauh
I have 2 rabbits. The first one, a Dutch doe 3 yr old, used this cage with the door open for over a year. I gave her everything she needed outside the cage and she gradually quit using it. The new rabbit is a large black doe that I named Midnight. When I put her in the cage with the used litter, she immediately started digging and eating choice bits which seem to get their value from being aged. There was no change after changing the litter so she only had her own excrement in the litter. She has no trouble getting cecotropes directly from the anus. Both rabbits have free access to timothy horse hay and during the growing season fresh horse grass. I have been limiting access to the preferred 3rd cut small pet timothy as an attempt at weight control but this digging and eating occurred when all 3 of the above were freely available. Both eat some of each when all are offered free choice, fresh grass is only part of the desired mix.
The digging box shown is the first design I have liked. I will give Journey one in the basement but Midnight only does things for a purpose and I would have to add on to her cage to give her one. She likes her cage and refuses to leave it for any reason. As to stress, when Journey was my only rabbit, she would sometimes go into her cage and start chewing on the steel wire. When I first got Midnight she would also chew on the wire but now neither do so. They can see and smell each other and that seems to be beneficial. I had intended for them to live together but I have not been able to get them to live in close contact. During the first month that I had Midnight, she was in her hide as much as possible but now only uses it a passage way except briefly if she is truly frightened.
It is hard to photograph this hay and litter room, 2 photos can be found at https://c.mail.com/@889354915774734947/qLyJmguTQv2jEID9KtHD6Q https://c.mail.com/@889354915774734947/XekZGC2GQ6ieWTLD76tnYQ Her big room is 60 by 30 inches and contains horse hay in one corner, fresh grass next to it, other greens I give her near by, and boles for water and food pellets. She comes near the front when she wants to be petted and toward the back when she wants to be alone.