They do need minerals that can not be found in nectar, they are probably finding little water drops or puddles where the minerals have leeched out of the limestone and are drinking from them.
Hello from Wisconsin and welcome Fiddlemama to this great site. From what Toni said I would think that is what they are doing. I love butterflies. Glad you get to see so many everyday. Are they different butterflies or the same species?
Welcome to GardenStew fiddlemama. You brought up a great subject to discuss in your first post. :-D Toni is right about butterflies requiring more than just nectar in their diets. They also need to get enough minerals and salts in their diet to help in their reproductive cycle. Males, especially, require these to produce sperm. Some of the sources of these trace elements include urine, dung, standing water, sand, mud and limestone all of which can provide them with valuable minerals. This feeding habit is called "puddling." If there's no water around, a butterfly may regurgitate into the soil and then drink in the hope of retrieving vital minerals and salts. This is why you'll sometimes see quite a few butterflies drinking from very shallow, still water. This water has absorbed minerals from the soil underneath it, and the butterflies need these minerals to supplement their diet. Now and then some of the butterflies will fly away from a puddle and return to it a few seconds later. This may disturb the water, bringing more minerals to the surface. You could make a puddling area in a small dish in your garden with a mixture of sand, Epson salts or table salt, and water. A mixture of sand, water, and manure also works well. Place the dish in a sunny location in your garden and keep moist. The butterlies will certainly thank you for it.
