My sow digs up reshes and takes them in to help her structure her bed, bedroom and wind break. I don't interfere, just give her more straw when she looks like she needs some.
She likes to make a mattress of mostly straw, with high sides (higher than her) made of straw and reshes, a windbreak inside the door ditto, and then a pile of just fresh straw to push her head into.
Her ark has a wooden floor raised off the ground. The metal arc is corroding a bit in places where it joins the floor; she blocks these gaps with trampled soil and reshes. I was clearing some of this out one day and she made it very plain that she'd spent some time doing this and did not appreciate my interference!
She also makes an entrance mat by strewing reshes in front of the door, so that she (and I) can get in and out of the ark without sinking into the mud.