Butterflies Grouped Together

I snapped this picture at a resting area in a nearby national forest. I believe there are three different species of butterflies here. Why are they grouped together like this? There did not appear to be any water on the ground.

—Bill Campbell, Kingsport, Tennessee

Tom: This activity is called "puddling" and it's a common sight, especially during early spring in wooded areas of the Appalachian Mountains and elsewhere. Newly emerged male butterflies of several species will gather at moist spots to sip minerals and salts from the wet soil. Although you mentioned that there didn't appear to be water on the surface, all the butterflies need is a slightly damp area from which to extract the collected minerals. In your photo, the two dominant species are tiger swallowtails and pipevine swallowtails. They have also been joined by one or two spicebush swallowtails and a few duskywing skippers.