I also think it is the heat pal,as I have a two tier huch too, my hob is on his own in the top one and has not pulled any of the bedding out of his nest box but I have four jills at the bottom which have pulled all of the bedding out, I am thinking because they were too hot from the heat off each other plus the bedding, were the hob is on his own so just has the heat from the bedding, if this makes sense?