I've read about this but never witnessed it. Butterflies have often been recorded alighting and feeding on fresh animal droppings. Dog mess is apparently attractive to the white admiral and I seem to recall reading that the late and eccentric lepidopterist Baron de Worms carrying dog excrement with him on a field trip to catch woodland butterflies - no need to bother these days as every green space has plenty kicking around for nothing much of which can easily be picked up in the tread of your boot!
With my Olympic obsession waning at the thought of watching dressage I made a quick evening visit to the area around Beaufort Ponds in preparation for a forthcoming talk for Gwent Wildlife Trust. As can be seen from the photos up to eight green veined white were observed feeding on a fresh pile of horse dropping, taking advantage of its mineral content. Elsewhere there were 17 Canada goose on Beaufort Pond and a 100+ swift high above the Heads of the Valleys road that soon disappeared eastwards. Now back to rhythmic gymnastics.