For many years now I have noticed that each spring, as the Eastern Tiger Swallowtails invade the valley, something strange takes place at the end of my driveway. Large groups of butterflies can be seen together milling about and flapping their wings gently. There can be as little as one or two or over fifty. Often I have to get out of my car to shoo them away so I don't run over them. I have always wondered what in the world was happening.
Finally, last year I did a little research on the Internet and found out it is a phenomenon called "Mud-Puddling" or just "Puddling" on some sites I checked. Basically, what the butterflies are doing is getting liquid nutrients from the wet soil in that area. Here is a bit of info from Wikipedia.
"Males participate in a behavior called puddling, in which they congregate on mud, damp gravel, or puddles. They extract sodium ions and amino acids from these sources which aid in reproduction. Males that puddle are typically fresh, and puddle only for their first couple of days. Females will occasionally puddle, but do not form congregations."