Bountiful Butterflies

I took this photo last year in my neighbor's yard. I have never seen so many butterflies together. Can you tell me what these are and explain more about them?

—Nora Glick, Waterloo, Iowa

Tom: These are painted ladies, a species most common west of the Mississippi. These fliers build up huge populations that migrate in large numbers, usually from the south to north. You probably witnessed one of these migrations. This species feeds on a number of plants as caterpillars, but are particularly fond of thistles.