Actually, they are just sort of playing in the box and messing (or "muffing") up the scrape in the box. Imagine digging a hole in your garden for a plant and the neighbour's kid, cat, dog, squirrel comes and knocks the sides a bit or plays in it and the nice neat sides/bottom/edges you made are now kind of "muffed" up.
Can't believe you googled it BallyWing, now you are going to make me paranoid about my words!! Actually, I tend to put quotations around words that are made-up, if that helps.
And yes: box = west-side nestbox; ledge = east-side nestledge