One observation today suggested these are representatives of the migratory generation: A male and female were nectaring side by side, with absolutely no mating behaviors, including chasing or aggression. They just ignored one another.
Clusters of migrating monarchs have been seen. The Journey North website reported a "roost" of migrating monarchs at the north end of the Pheasant Branch Conservancy in Middleton, WI. Over a hundred were estimated huddling on three oak branches.
See this excellent article by Dr. Karen Oberhauser and Jacob Swanson.