Look up over a New Glarus prairie, roadside or backyard on a warm September afternoon and the orange-and-black butterfly moving steadily southwest may be doing much more than drifting from flower to flower. It may be headed for Mexico.

Wisconsin's annual monarch migration is well underway. The largest wave typically moves through the Upper Midwest earlier in September, but monarchs continue to pass through southern Wisconsin as the migration front advances south. Journey North reported that the leading edge reached northern Oklahoma and Texas on Sept. 22, while Wisconsin observers were still reporting monarchs and roosts farther north.

The scale can be startling when the butterflies gather. A Journey North observer in Green Lake estimated about 1,000 monarchs on Sept. 6 in trees around a field of blooming buckwheat. Three days later, an observer in Arlington, north of Madison, estimated 596 monarchs roosting on the south-facing sides of trees. Those are individual community-science observations rather than statewide population counts, but they offer a glimpse of how concentrated the migration can become.

The travelers are unusual even by monarch standards. Karen Oberhauser, a University of Wisconsin-Madison professor emerita and longtime monarch researcher, wrote for the Wisconsin Monarch Collaborative this month that monarchs emerging after about Aug. 15 respond to shortening days and the changing angle of the sun by delaying reproduction, storing fat and beginning the long trip to central Mexico. Their parents and grandparents lived very different lives; this late-summer generation is built to migrate.