Hedgehog Hoard

Back in August I introduced you to our Hedgehog regime. This is the feeding station in the corner of the garden (not near the hedgehog home).
 
On a number of nights we have found some stones in the near empty food bowls, but none of the hedgehog societies/groups could give us any idea of why, other than they were perhaps accidentally dragged in by one or more of the hogs.
 
Last night there was a spiky friend waiting for his food. This morning we found the food hardly touched and all these stones piled on top . . .  This is no accident!
 
Does anyone have an idea of what is going on???