What is the appeal of these hay sacks?

When I finished planting the rest of the greenhouse, I had to move out the assorted boxes and junk that have accumulated in there.These are the sacks of hay and poop that I bag when I clean out the hen coops, on their way to being carted to the garden for mulch.For some reason, the hens are always fascinated by these bags, jumping on them and scratching in the top.Jacques the rooster, the weirdo, squeezed himself in between the bags, and then got himself stuck.