It's curious how many crotal bells are turned up by metal-detectorists and the conclusion must be that many domestic animals, in the past, must have worn them. Which means they must have been very closely shepherded. Probably by a child, in the days before widespread schooling. The largest threat to monuments is people, first and last, and I concur that sheep are the worst by puddling round stones, using them as rubbing posts, which are then undermined and slip or topple.